May the Force Be with You: The Legal Classification of Intervention by Invitation

It is a truth universally acknowledged that states can consent to the military presence of other states on their territory. This is better known as intervention by invitation. Yet many issues surrounding this concept remain unclear or are too easily accepted, e.g. its name and its place within the rules of jus ad bellum. This article seeks to clarify and resolve these issues. First, an analysis is conducted into what the two terms intervention and invitation actually entail. The term intervention is contrasted with the use of force and the entire concept of intervention by invitation is differentiated from collective self-defence. It is concluded that the threshold of force has been met and thus the focus should be placed on the rules regulating this field of law, rather than the rules of non-intervention. The concept would be more aptly labelled as the use of force by invitation. Second, this article examines where intervention by invitation finds its place in relation to the prohibition of the use of force. Alternative perspectives are investigated encompassing the scope of Article 2(4) UN Charter and the circumstances precluding wrongfulness under the rules of state responsibility, of which consent is of particular relevance here. This article concludes that intervention by invitation falls outside the scope of Article 2(4) as the force is not used within international relations. The prohibition of the use of force therefore does not apply to intervention by invitation. Consequently, an action of intervention by invitation is legal.


Introduction
Around three decades ago, Louise Doswald-Beck proclaimed 'there is certainly no doubt that a state can legally send troops to another state upon invitation for certain limited military purposes'. 1 With the recent interventions by invitation by France in Mali, 2 the US-led coalition in Iraq, 3 Saudi Arabia in Yemen, 4 Russia in Syria, 5 and most recently by the Economic Community of West African States (ECOWAS) in The Gambia, 6 the issue has not only become topical again, but also controversial. 7 Intervention by invitation has accordingly attracted increased attention among international legal scholars. 8 Most of these academic discussions have centred on issues 9 such as who represents the government of a state 10 and whether an intervention by invitation is allowed when the state is engaged in a civil war. 11 While these matters are interesting and certainly deserve further investigation, the scholarly discussions merely glance over or even ignore the more fundamental questions concerning intervention by invitation. These fundamental issues concern the concept's name and thus what it actually entails, as well as where it finds its place within the rules of jus ad bellum. The very purpose of this article is to clarify and resolve these underlying issues.

General Assembly and Institut de Droit International (IDI) Resolutions, and reports from the International Law Commission (ILC).
Section 2 defines the concept of intervention by invitation. To this end, the terms intervention and invitation are clarified. The term intervention is contrasted with the use of force and the entire concept of intervention by invitation is differentiated from collective self-defence. In this regard, the article clarifies that intervention by invitation relates not only to actions against non-state actors, but also against states, as long as the fighting is limited to the inviting state's territory. It furthermore argues that intervention by invitation might not be its proper name, as it does not deal solely with an intervention, but actually deals with a use of force, which is a separate (but related) notion under international law to which different rules apply. A more appropriate name is therefore the use of force by invitation.
Section 3 addresses how intervention by invitation fits into the jus ad bellum rules. Article 2(4) UN Charter contains the prohibition of the use of force. Intervention by invitation could fall outside the Article's scope altogether. Two potential reasons for this conclusion are provided. Alternatively, it could be an unwritten exception to the prohibition. Finally, it could violate the prohibition but still be justified by consent as a circumstance precluding wrongfulness under the rules of state responsibility. The latter option is discussed in a separate section of the article (Sect. 4), as it requires a more thorough analysis of the intricacies of state responsibility. 12 These are whether the circumstances precluding wrongfulness act as justifications or excuses, what the circumstance of consent entails, and whether the prohibition of the use of force is in fact a norm having a jus cogens character, the wrongfulness of which cannot be precluded.
The article compares these four different approaches in Sect. 5 and concludes that the prohibition of the use of force does not apply to intervention by invitation as the force is not used in international relations. The notion of consent has been built into that part of the primary norm of the prohibition. Thus, when an invitation has been issued, no violation of the prohibition occurs. It is not purported that this conclusion is new or ground-breaking. Yet, by thoroughly exploring and comparing all these options, the conclusion carries greater weight and that is significant in its own right.

The Intervention
The term intervention by invitation has a number of synonyms, such as military assistance upon request, consent to the use of force, and foreign armed intervention. Yet academic texts often fail to provide a definition. Recourse is therefore had to two 12  texts that do clearly define the concept. According to Georg Nolte, '[t]he expression "intervention by invitation" is mostly used as a shorthand for military intervention by foreign troops in an internal armed conflict at the invitation of the government of the State concerned'. 13 The IDI Resolution of 2011 states that '"[m]ilitary assistance on request" means direct military assistance by the sending of armed forces by one State to another State upon the latter's request', 14 the objective of which is to 'assist the requesting State in its struggle against non-State actors or individual persons within its territory'. 15 The following common grounds can be deduced from these definitions. First, the conflict is taking place on the inviting state's territory, where that state is fighting against a non-state actor and is thus involved in a non-international (or internal) armed conflict. 16 Consequently, these definitions exclude international armed conflicts (two or more states in conflict with each other). 17 It will however be shown that an intervention by invitation can also occur against another state and thus will in fact include international armed conflicts. Second, intervention by invitation does not solely concern a political or economic intervention, but a military intervention with armed forces or troops. This indicates that it concerns a use of force and not solely an intervention. 18 These two terms are often used interchangeably and rather indiscriminately in academic writings. 19 Both therefore need to be explained and their differences demonstrated to properly define the concept of intervention by invitation. The distinction between the two terms (and thus the use of the correct term) is crucial, as the principle of non-intervention applies when talking solely of an intervention. Only when a use of force is at stake does the prohibition of the use of force come into play.
Assembly resolutions have incorporated the principle by stipulating that 'no State has the right to intervene, directly or indirectly, for any reason whatever, in the internal or external affairs of any other State'. 21 An intervention can take different forms, for example a political intervention, an economic intervention, or a military (or armed) intervention. 22 Not all interventions are illegal, however. In the Nicaragua case the International Court of Justice (ICJ or the Court) clarified that to be wrongful the intervention must use methods of coercion. 23 These are 'particularly obvious in the case of an intervention which uses force' 24 and '[t]hese forms of actions are therefore wrongful in the light of both the principle of non-use of force, and that of non-intervention'. 25 Thus, a clear link can be identified between the prohibition of intervention and the prohibition of the use of force 26 : if a state uses force illegally, it violates both prohibitions. 27 Every use of force therefore also constitutes an intervention under international law, but not every intervention constitutes a use of force (only those interventions that use force do). 28 The question then remains what kind of force is covered by the term 'use of force'.
A proposal to include economic measures into the notion of force under Article 2(4) UN Charter was rejected during its drafting process. 29 It is generally accepted that the term only refers to armed force. 30 Despite this agreement among scholars, disagreement is just around the corner. Many authors try to distil additional elements to define 'use of force'. In his extensive work 31 on the prohibition of the use of force, Olivier Corten 32 submits there is a certain threshold that needs to be met for a forceful action to fall within the scope of force in Article 2(4). 33 This threshold is determined by the criteria of gravity and intent. For gravity, the place and context 21 Kohen (2012), p. 161. 28 As already exemplified by Jamnejad and Wood when they state '[t]he prohibition of the threat and use of force […] is the most significant aspect of non-intervention', Jamnejad and Wood (2009), p. 359. 29 Randelzhofer and Dörr (2012), p. 209; see also Shaw (2017), p. 855;Ruys (2014), p. 163;, p. 53. 30 Ruys (2014), p. 163;Dinstein (2017), p. 90;, pp. 53, 55. 31 Ruys rightly observes that 'Corten's work is arguably the first in-depth analysis of customary practice relating to the scope of UN Charter Article 2(4)', Ruys (2014), p. 159, fn. 2. 32 Corten (2010). 33 He concludes this based on an analysis of several examples of military force and police measures on land, at sea, and in the air and states' reactions to these actions, see Corten (2010), pp. 52-66. matter. 34 For intent, 35 the focus is on 'forcing the will of another State' 36 or where 'one State intends to defy another'. 37 For Corten a clear example of the threshold being crossed is when 'combat breaks out between the forces of two States'. 38 Tom Ruys, 39 on the other hand, contests the existence of a de minimis threshold within the term use of force. 40 More specifically, he contests the existence of a gravity threshold, thus arguing that all uses of force will fall under the scope of the prohibition as contained in Article 2(4), however small or large-scale the incursion is. 41 He does agree with Corten, however, that intent is in fact important. 42 If armed forces accidently cross a border, thus with no intent to force the will of the state they now find themselves in, this is not a use of force. However, if armed forces cross a border with the intent to harm the state, but no shots are in fact fired, this would constitute a use of force. 43 Alternatively, it can be submitted that there are no further requirements to classify a use of force, other than it must concern an actual use of armed force.
Whichever approach one might be inclined to follow, an intervention by invitation will always meet the threshold of all of these approaches. Following the reasoning of Corten, 44 a use of force is only classified as such if it is sufficiently grave and if there is an intent to defy another state. Thus, if a use of force meets Corten's threshold, it will also fall within the approach advocated by Ruys as he does not favour a gravity threshold, but does put forward the need for intent. As for the gravity threshold, this is met by interventions by invitation. A state is invited in with its armed forces and is engaged in military combat with the opposition (whether that is a state or a non-state actor, see further Sect. 2.1.2). Think of the Russian forces in Syria or the French forces in Mali. The invited state is not involved in merely undertaking police measures, such as making arrests, or minor border incursions. 45 It is involved in actual armed activities. The gravity threshold is thus met. 34 Corten (2010), p. 73. 35 The ICJ has referred to the importance of intent in two of its decisions, namely Nicaragua case, supra n. 20, p. 120, para. 231 and Oil Platforms (Islamic Republic of Iran v. United States of America), Judgment, ICJ Reports 2003, p. 161, at pp. 191-192, para. 64. However, in both of these cases the Court was trying to establish whether an armed attack had taken place, not a more general use of force. See also Ruys (2014), pp. 172 et seq. 36 Corten (2010), p. 77. 37 Ibid., p. 78. 38 Ibid., p. 87. See also Ruys (2014), p. 188, who states that armed confrontations between states lead to the application of Art. 2(4). 39 Ruys (2014). 40 Ibid.,pp. 171,188,209. 41 Ibid.,pp. 159,171,209. 42 Or, as Ruys refers to it, deliberate recourse to lethal force, see Ruys (2014), pp. 160, 171-177, 209. 43  , pp. 75-80. 44 It is telling that Corten does not explicitly address this threshold when he deals with intervention by invitation, see Corten (2010), pp. 249-310. 45 See for a discussion on these actions, Corten (2010) An intervention by invitation also fulfils the requirement of intent. The armed forces invited into the state clearly have the intent to use force; that is in fact precisely the reason why they are invited in. The territorial state needs help in its struggle against the opposition. The invited foreign troops are thus present precisely with the intent for them to use force.
In conclusion, an intervention by invitation will always meet the threshold of a use of force. The concept therefore better fits within the rules of the use of force, rather than merely the rules on intervention. It would thus be more fitting to talk about a use of force by invitation rather than an intervention by invitation, as the latter has a much broader scope than the former. Nevertheless, for the purposes of consistency the original term will be used in this article. This classification also leads to the application of the appropriate rules, namely the prohibition of the use of force, as a kind of lex specialis. This does not make any pronouncements on the legality of the intervention nor on the legality of the use of force. Stating that it concerns a use of force merely results in the rules on the use of force coming into play, which will subsequently determine whether the action is lawful. 46

Collective Self-Defence
An additional predicament in defining the concept of intervention by invitation is how it differs from a use of force within the notion of collective self-defence. These two concepts seem to be very similar, as both deal with the situation where a third state is invited by a state under attack to use force. 47 In order to properly define intervention by invitation it is thus critical to differentiate between these two concepts.
Collective self-defence is a recognised exception to the prohibition of the use of force, as it is contained in Article 51 UN Charter and accepted as part of customary international law. 48 Intervention by invitation is not so easily placed in relation to the prohibition of the use of force. 49 Furthermore, two key differences between the concepts can be identified: whether an armed attack is required and whether the force is exercised within or outside the inviting state's territory.
To exercise the right of collective self-defence, the requesting state must be the victim of an armed attack. There is no need for an armed attack to occur to trigger the right to intervention by invitation. A lower threshold for such a use of force can therefore be observed. In the Nicaragua case and reiterated in the Oil Platforms case, the Court noted different forms of uses of force: there are grave forms which constitute an armed attack, and 'other less grave forms', 50 such as mere frontier 46 Corten (2010), p. 125. 47 See for the requirements of collective self-defence, Nicaragua case, supra n. 20, pp. 103 and 105, paras. 195 and 199;Greenwood (2011), paras. 35-38. 48 Nicaragua case, supra n. 20, p. 27, para. 34 and pp. 102-103, paras. 193-194. 49  incidents, 51 which constitute uses of force. 52 Thus, an armed attack is graver both in scale and effect than a use of force. 53 The difficulty with the notion of an armed attack is the source or author of the attack. 54 It is well established that a state can conduct an armed attack which is necessary to trigger the right of self-defence. 55 However, despite the rapidly evolving literature in this field, there is a lack of clarity when the armed attack emanates from a non-state actor. 56 Following the two definitions provided at the beginning of this section, the source of the hostility for an intervention by invitation is a non-state actor. The second key distinction relating to territory will however challenge this traditional interpretation and make clear that the source/author can also be a state. Two illustrative situations are presented before turning to the underlying legal theory.
Force as part of an intervention by invitation is used within the requesting state's territory. If state A is attacked on its own territory by state B or a non-state actor, then state A can invite state C to use force against state B or the non-state actor, but this force can only be used on the territory of state A. In contrast, force as part of collective self-defence can be used outside the requesting state's territory, namely within the state's territory that attacked the requesting state in the first place. Thus, if state X is attacked by state Y, it can defend itself by counterattacking state Y on state Y's territory, provided that the requirements for individual self-defence are met. 57 If state X requests state Z to assist it, then state Z can not only use force on X's territory to repel state Y, but also on Y's territory, provided that the requirements for collective self-defence are met. Thus, when a state is attacked on its own territory, it does not need to rely on self-defence to strike back. 58 The requirements for self-defence do have to be met, however, when the force is to be used outside the attacked state's territory. 59 This follows the general idea of the Charter: in the absence of an armed attack and thus without a claim of self-defence, the state affected can 'respond only by means falling short of the use of cross-border force'. 60 51 Nicaragua case, supra n. 20, pp. 103-104, para. 195. 52 59 See Gray (2018), pp. 176-177 for a list of instances of states relying on the notion of collective selfdefence, which is quite short. In most of these listed instances, the force was not used on the attacking state's territory, so these are strictly speaking not examples of collective self-defence proper. 60 Randelzhofer and Nolte (2012), p. 1401, para. 6.
The complication here is of course when the attack comes from a non-state actor operating from another state's territory. 61 The right of self-defence can traditionally only be exercised against a state, so the attacked state would not be allowed to enter the other state's territory to attack the non-state actor located there. The attacked state would be allowed, however, to counter the attack of the non-state actor on its own territory as this falls under the concept of intervention by invitation (if assistance is requested). Current developments, 62 such as the unwilling or unable doctrine 63 or the plea of necessity, 64 try to place these forceful actions against the nonstate actor operating from a foreign territory within the spheres of legality.
Two situations are thus identified. First (as exemplified by states X, Y, and Z), a state uses force outside its own territory. In order to do so, it needs to be able to rely on the right of self-defence, either individual or collective. Accordingly, an armed attack is required to occur, which has been conducted by a state (or potentially by a non-state actor). Second (as exemplified by states A, B or a non-state actor, and C), a state is using force on its own territory. If the state invites another state to provide military assistance, then this falls under the notion of intervention by invitation. In this second situation, it is irrelevant whether the state is attacked by another state or by a non-state actor and whether that attack amounts to an armed attack proper. Crucially, the fact that the author of the attack can also be a state contradicts the abovementioned definitions of intervention by invitation.

The Invitation
Having ascertained the 'intervention' part of the concept of intervention by invitation, attention now turns to the 'invitation' part to see what requirements such a request must fulfil.
Following Nolte and the IDI Resolution once more, the invitation requires 'demonstrable consent by the highest available governmental authority' 65 and it must be 'reflecting the free expression of will of the requesting state and its consent to the terms and modalities of the military assistance'. 66 Again, a few elements can be deduced. First, the invitation must come from the state itself, not from a non-state actor (for example, the opposition fighting the government of the state). 67 The intervening state is thus only allowed to use force on the side of the inviting state, not on the side of the non-state actor. This is in line with paragraph 246 of the Nicaragua judgment, where the Court stated: [I]t is difficult to see what would remain of the principle of non-intervention in international law if intervention, which is already allowable at the request of the government of a State, were also to be allowed at the request of the opposition. 68 This can also be concluded based on the reactions of the Security Council as analysed by Christine Gray and the fact that on those rare occasions when states have in fact supported the opposition militarily, they have done so neither with their own troops nor in the open, apparently being fully aware that what they were doing was not in line with public international law. 69 A second element of the nature of the invitation is that it must come from the highest available governmental organ. 70 This means that an invitation from the military will not suffice, as it does not normally form part of the official government. 71 Third, it must be provided freely and not under pressure or through coercion. 72 Fourth, the intervention needs to stay within the limits of the invitation. 73 Others have distilled additional elements, namely that the intervention cannot violate another norm of jus cogens (arguing that the prohibition of the use of force itself is such a norm) 74 and that it must be produced prior to the intervention, either ad hoc or through a treaty. 75 On this latter notion some have however argued that the invitation can only be extended ad hoc and not as a blanket authorisation through a prior treaty as this would represent an expression of the will of the government at that time, which is not necessarily the same as that of the current government. 76 This line of argumentation does not seem to be in accordance with the Tinoco arbitration ruling 77 where a succeeding government was held to be bound by the actions of the former government. It is submitted here that such a previous invitation, also via a treaty, is still valid up until the moment the (new) government clearly states the opposite. 78 Having defined both the intervention and the invitation and having subsequently suggested use of force by invitation as a more fitting name, this leads to the utilisation of the rules on the use of force. The next sections will discuss the potential relation of intervention by invitation to these rules.

The Relation Between Intervention by Invitation and Article 2(4) UN Charter
This and the next sections determine where intervention by invitation finds its place in the wider notion of public international law. As has already been established, the correct rules to take into account are not the ones on non-intervention, but the rules on the use of force, since intervention by invitation meets the threshold for force. 79 By focussing on the rules on the use of force, this does not indicate by any means that an intervention by invitation is also caught by it. All that has been established so far is that an intervention by invitation actually constitutes a use of force. Now it remains to be seen whether intervention by invitation is caught by the rules of the use of force, thus what its relation is to the prohibition of the use of force. The prohibition of the use of force is contained in Article 2(4) of the UN Charter, which the Court has pronounced to be one of the Charter's cornerstones. 80 It also reflects customary international law. 81 Gray has accurately observed that it is 'clear' 82 that Article 2(4), since it was adopted just after the Second World War, was meant to deal with conflicts between states and not with conflicts within a state. 83 As intervention by invitation falls within the latter category, its classification in relation to the prohibition has for that reason been challenging. The divergent views can be divided into four categories: (1) intervention by invitation falls outside the scope of Article 2(4) because the force employed by the invited state is not used within international relations; (2) it falls outside the scope of Article 2(4) because the force is not used against the territorial integrity or political independence; (3) it falls within Article 2(4), but it forms an exception to the prohibition contained therein; and (4) it falls within Article 2(4), but the wrongfulness is precluded due to consent being given. These different approaches shall be presented in turn; the first three directly below and the fourth in Sect. 4. Section 5 then compares all these approaches and will identify the correct legal approach. 79

The Force is not used Within International Relations
Article 2(4) makes clear that states are to refrain from using force not simply in every situation, but in their international relations. This implies that if the force used cannot be classified as taking place in the state's international relations, that force is not prohibited since it falls outside its scope. 84 It is argued that using force upon an invitation from an attacked state within that state's territory is precisely this: not using force in international relations. This would be irrespective of whether the attacking party is another state or a non-state actor. States A and C, the attacked and invited states respectively, do use force, but do not do so in their international relations. State A is attacked on its own territory and wants to strike back on its own territory. It does not therefore use force in its international relations as the force is limited to its own territory. State C is invited by the state under attack (state A). It does not use force in its international relations either, as it is acting with the permission of the attacked state to use force upon the latter's territory. The international relations of the attacked and the assisting state (states A and C) are not affected by such an intervention, 85 as states are sovereign and can therefore dispose of their territory as they see fit. 86 For this classification, it is thus irrelevant whether the attacking party is a state or a non-state actor. Furthermore, if the attacking party is a state (state B) its use of force will fall within the scope of the prohibition, as it uses force within another state and against another state.
The result of this approach would be that this type of use of force (i.e. intervention by invitation) does not fall under the prohibition as contained in Article 2(4). It therefore falls outside this prohibition and this use of force is consequently allowed. 87

The Force is not used Against Territorial Integrity or Political Independence
The second approach focuses on the latter part of Article 2(4) and proposes that the notions of territorial integrity and political independence are not violated by an intervention by invitation. The result of this second approach is the same as the first, namely that this type of the use of force falls outside the scope of Article 2(4) and is therefore allowed. Yet the road taken to reach that conclusion is slightly different, although somewhat similar. It is argued that the territorial integrity and political independence of the inviting state are not violated, precisely because the intervening state is acting with the permission of the inviting state. 88 Wippman (1996), p. 210 (even though he also refers to it as a justification at times). 85 Corten (2010) government as the direct representative of the state provides consent to enter its territory with military forces, this does not violate these two elements. The use of force therefore does not violate the prohibition contained in Article 2(4) and is therefore allowed.

Intervention by Invitation Forms an Exception to the Prohibition
The third approach suggests that intervention by invitation constitutes an exception to the prohibition of the use of force. The prohibition is violated, yet intervention by invitation forms an exception to the rule.
One could argue that any use of force conducted by a state outside its own territory, even if only within the inviting state's territory, is in fact force used in international relations. 89 It can be added that this could also (or alternatively) constitute force used against the territorial integrity or political independence of the inviting state, thereby also falling within the realm of Article 2(4), as these are at the very least compromised by the intervention. Following this analysis, the intervention by invitation therefore violates the prohibition of the use of force.
Under these circumstances, intervention by invitation could form an exception to the prohibition of the use of force, 90 although the Court has never dealt with it as such. 91 The exceptions to the prohibition are clearly laid down in the Charter, namely Security Council authorisation and self-defence, 92 and there is no reference to intervention by invitation in the Charter. It cannot therefore be seen as an exception under treaty law. 93 However, a customary exception could be another option. State practice would need to be analysed to determine whether those states engaged in an intervention by invitation actually conceive of their actions (have the required opinio juris) as an exception to the prohibition.
The three approaches discussed thus far all directly relate to the rules on the use of force. Section 4 addresses another approach from the perspective of the rules on state responsibility. 89 See Tanca (1993), pp. 20-21, 47. He discusses this possibility but disagrees with it in favour of the approach where the prohibition only prohibits force used against another state, not merely force used outside a state's territory. 90 See for example Fox (2015), pp. 816, 819 (even though he expressly refers to the notion of consent earlier on in his text) and De Wet (2015), p. 980 (even though the surrounding analysis discusses the elements of political independence and territorial integrity). 91 Kreβ (2015), p. 577. 92 See ILA Sydney Conference, supra n. 10, pp. 7, 18. 93 Ibid., p. 3.

Consent as a Circumstance Precluding Wrongfulness for the Use of Force
The fourth and last approach to classifying intervention by invitation within the framework of the prohibition of the use of force leads to the concept of consent as part of the circumstances precluding wrongfulness under the rules of state responsibility. In the first three approaches, it was unnecessary to even consider consent, because the conduct is either not classified as being illegal, since it falls outside the scope of Article 2(4), or it forms an exception to the rule. Yet if the intervention can be said to breach Article 2(4), the fact that consent was given (the invitation was issued) changes the situation. To fully comprehend this approach a thorough and extensive discussion on a number of issues is required. These issues range from circumstances precluding wrongfulness and whether they act as justifications or excuses, to consent more specifically, and to whether the prohibition of the use of force is a norm of jus cogens (as the wrongfulness of these types of norms cannot be precluded). This fourth approach is therefore dealt with separately in this section.

Circumstances Precluding Wrongfulness: Justifications or Excuses?
ARSIWA contains the rules on state responsibility. 94 One needs to bear in mind that these Articles are not contained in a treaty (or any other type of binding legal instrument) and the form of the Articles was purposefully left open, as requested by the ILC. 95 Yet they are now said to be largely reflective of customary law. 96 These rules on state responsibility are generally called the secondary rules of international law. 97 They will determine whether an internationally wrongful act has been committed. The primary rules, on the other hand, determine the substantive obligations. 98 The terminology of primary and secondary rules is unfortunate, however, 99 as it was never meant as anything more than a methodological approach or an 'extremely useful' 100 distinction. Finding that a state has committed an internationally wrongful act under these secondary rules is however not the end of the story; there could be mitigating factors applicable to the situation. These factors are properly referred to as circumstances precluding wrongfulness. 94 Crawford (2002), p. 877; see also p. 879 where he explains that the distinction 'is to some extent a functional one, related to the development of international law rather than to any logical necessity'.
Circumstances precluding wrongfulness are included in Chapter V of ARSIWA and act as defences. Defences can act in two different ways: as justifications or as excuses. 101 To put it differently, they can either preclude wrongfulness or mitigate against responsibility. If these circumstances act as justifications, the act constituting a breach of an international obligation is no longer classified as such, i.e. there is no breach to speak of and no illegal conduct has taken place. This means that the wrongfulness is precluded. If these circumstances act as excuses, it is the responsibility that is precluded. An illegal conduct still exists, and thus a breach, but due to the specific circumstances the state cannot be held responsible. 102 The result might, practically speaking, be quite similar in both situations, namely that the state is absolved, but legally speaking the consequences are rather divergent: with a justification there will be no wrongfulness, with an excuse there is still a wrongful conduct, but no responsibility.
In its Commentary to this section of ARSIWA, the ILC stated: Chapter V sets out six circumstances precluding the wrongfulness of conduct that would otherwise not be in conformity with the international obligations of the State concerned. The existence in a given case of a circumstance precluding wrongfulness in accordance with this chapter provides a shield against an otherwise well-founded claim for the breach of an international obligation. 103 This seems to indicate that the ILC considers all circumstances to act as justifications, as without the circumstances there would be a breach, meaning that the wrongfulness is precluded. 104 This was indeed the opinion of the Drafting Commission of the ILC led at the time by Special Rapporteur Roberto Ago. 105 Later, Special Rapporteur James Crawford raised the issue again, but no conclusion was reached. 106 Instead, it was left to future discussion and the further development of the circumstances. 107 Some of the circumstances contained in Chapter V ARSIWA are generally accepted to act as excuses, e.g. necessity or distress. 108 The analysis will now turn to the circumstance precluding wrongfulness that is specifically at stake in this article, namely consent.

Consent
'In general, governments have the capacity to consent on behalf of the state and opposition forces do not. Indeed, "the very ability to make such a request reinforces the inviting state's authority".' 109 This notion is at times referred to as volenti non fit injuria, i.e. to the willing no injury comes. This indicates that if a state has consented to a certain act, it cannot be injured by that act in a legal sense. 110 Examples which show that providing consent occurs rather often in inter-state relations include passage through internal waters and the location of facilities on the consenting state's territory. 111 It is to be determined how consent can be validly granted and if it acts as a justification or an excuse. 112 Firstly, to establish the validity of consent, several elements can be considered. 'Valid consent' and 'that the act remains within the limits of [the provided] consent' 113 are all the elements contained in Article 20 ARSIWA. More elements for consent have been developed, however. For example, different people may be entitled to provide consent in different types of situations. 114 In addition, it might be quite problematic to determine who has the authority on behalf of the government to issue the consent, particularly in times of civil war. 115 Furthermore, it becomes clear that consent 'must be freely given and clearly established', 116 it 'must be actually expressed by the State rather than merely presumed' 117 and it 'may be vitiated by error, fraud, corruption or coercion'. 118 However, the consent does not need to be public, as long as it is clear. The ARSIWA Commentary even takes note of a situation where the consent was issued implicitly 119 and in the DRC v. Uganda case the withdrawal of the DRC's consent to the presence of Ugandan troops was implicit, yet it was accepted by the Court. 120 Lastly, the conduct must fall within the limits set by the consent. 121 Specifically for providing consent in the use of force realm, this element is also found in the Definition of Aggression. 122 The armed forces present in the receiving state upon its agreement must leave after the termination of that agreement, i.e. the withdrawal of consent. 123 This raises another related point concerning the validity of consent, i.e., its withdrawal. Withdrawing consent is possible at any time, indicating the weight given to the sovereignty of the consenting state. 124 However, if it is issued after the conduct has already occurred (e.g. armed forces have already crossed the border), then it constitutes a waiver which is dealt with in Article 45 ARSIWA. With a waiver the consequences in terms of responsibility are waived, but there was legally speaking still a breach. 125 These elements all confirm what has been stated above concerning the nature of the invitation. 126 Secondly, to determine whether consent acts as a justification or an excuse, recourse is had to Article 20 ARSIWA, which incorporates the notion of consent as a circumstance precluding wrongfulness. Article 20 ARSIWA reads as follows: Valid consent by a State to the commission of a given act by another State precludes the wrongfulness of that act in relation to the former State to the extent that the act remains within the limits of that consent. 127 Thus, one state consents to the conduct of another state, whereas that conduct without this consent would be a breach of an international obligation. 128 The article literally states that the wrongfulness is precluded, not that the responsibility is mitigated. Consent therefore acts as a justification. 129 Yet only that particular conduct is consented to, meaning that the obligation still applies but is temporarily displaced in the particular situation. 130 The fact that consent is considered as a circumstance precluding wrongfulness was not evident for Special Rapporteur Crawford. 131 He sought to have the article deleted, 132 as he observed: But if consent must be given in advance, and if it is only validly given in some cases and not in other, and if authority to consent varies with the rule in question, then it may be asked whether the element of consent should not be seen as incorporated in the different primary rules, possibly in different terms for different rules. 133 Moreover, Crawford himself cannot identify any obligation which does not have the possibility of consent built into it, for which consent could still lawfully preclude the wrongfulness of a breach of that obligation. 134 Consent is thus either built into the primary obligation and if issued no wrongfulness arises, or it is not built in and it cannot therefore be raised at all. 135 Consequently, Article 20 indeed seems to be redundant.

Use of Force as a Norm of Jus Cogens
The conclusion that consent has no place within the circumstances precluding wrongfulness, especially in relation to the prohibition of the use of force, is further proven by another conceptual difficulty, i.e. the fact that the wrongfulness of peremptory norms cannot be precluded according to Article 26 ARSIWA. It must therefore first be discussed if the prohibition of the use of force constitutes such a norm of jus cogens, and if so, secondly, how this issue could be resolved.
First, the prohibition of the use of force is generally classified as a peremptory norm or, to be more precise, the prohibition of the use of aggressive or unlawful force is. This can be found in many documents, 136 such as the ILC Commentary to the Draft Articles on the Law of Treaties, 137 the ILC Commentary to ARSIWA, 138 the ILC Report on Fragmentation, quoting Brownlie and Aust, 139 and a Commentary to the UN Charter. 140 James Green, however, is critical of this position even though he does not exclude the possibility altogether. 141 He notes that 'the widespread uncritical acceptance of the prohibition as a jus cogens norm is concerning'. 142 This echoes Lianne Boer's 143 warning that in order to establish the majority opinion on a certain issue scholars often refer to the same sources, disregarding those sources that determine otherwise. The forthcoming illustrative list of jus cogens norms by the ILC Commission on peremptory norms of general international law (jus cogens) will be instrumental in this discussion. 144 In any event, given the authoritative sources cited above, it is concluded here that the prohibition of using aggressive or illegal force is a norm of jus cogens.
Second, given the peremptory norm status of the prohibition, it therefore seems troublesome to apply consent as a circumstance precluding wrongfulness. Nevertheless, the Commentary to Article 26 ARSIWA states something rather peculiar: 'in applying some peremptory norms the consent of a particular State may be relevant. For example, a State may validly consent to a foreign military presence on its territory for a lawful purpose.' 145 Thus, even though the ILC considers the prohibition of the use of force (or at least aggressive or illegal force) to be a norm of jus cogens, a state can still provide its consent to the presence of foreign military forces. This consent is precisely what makes the force not illegal as it is built into the primary rule. The reasoning is circular: aggressive or illegal force is prohibited as a norm of jus cogens, yet consent makes the use of force not aggressive nor illegal and it is therefore not prohibited. Thus, there is no problem in reconciling Article 26 with the issuing of consent for the prohibition of the use of force. 146 This also follows Crawford's earlier mentioned line of argumentation that Article 20 should have been deleted. 147

Comparing the Four Approaches
Having explored all of the four approaches it will now be ascertained which one works best to describe the legal relation between intervention by invitation and the prohibition of the use of force as contained in Article 2(4) UN Charter. The approaches will be considered in reverse order.
The fourth approach regarded the notion of consent as a circumstance precluding wrongfulness under the rules of state responsibility. The intervention only occurs because an invitation has been issued, thus consent has been provided. Consent can therefore act as a defence against the violation of the prohibition of the use of force. However, as consent acts as a justification and not an excuse, and especially since consent is built into the prohibition of the use of force and is therefore not a circumstance precluding wrongfulness proper, it is not appropriate to use this fourth approach in determining the relationship between intervention by invitation and Article 2(4).
The third approach-intervention by invitation acting as an exception to the prohibition of the use of force-does not seem to be befitting either, as no treaty exception consisting of intervention by invitation exists to the prohibition of the use of force. A customary exception could be another option. This would require an extensive study on the relevant state practice and opinio juris that is beyond the scope of this article. Nevertheless, this would lead to a clash of sources. Even if one were to accept the existence of a customary exception to the treaty norm of the prohibition of the use of force, difficulties arise from a doctrinal perspective. It ushers in a discussion on the possibility of such a customary exception leading to a deviation from a treaty norm without adapting the treaty itself. This is certainly problematic in its own right. Moreover, Article 103 UN Charter can be interpreted in such a way that the Charter does not only prevail over other treaty obligations, but also over other customary obligations. 148 Therefore, since there is no reference to intervention by invitation in the Charter, a customary exception cannot deviate from the prohibition contained therein.
Consequently, it seems more logical that intervention by invitation is not caught by the prohibition at all and therefore falls outside its scope, thus in line with the first or second approach. Many academic writings support this overall conclusion, namely that the intervention does not fall within the scope of the prohibition, that this prohibition is therefore not violated, and that the intervention is therefore allowed. However, all of these do so in a rather straightforward manner, for example by stating that the intervention is only illegal if the invited troops overstay their welcome or contravene the specifics of the invitation. 149 Additionally, it is purported that 'a State's use of force on the territory of another State with its consent involves no breach of Article 2(4) ab initio', 150 and that if a request for an intervention has been made the force used 'hardly goes against' 151 the territorial integrity or political independence of the state, and 'much less affects' 152 its international relations. Other authors 153 seem even less concerned with the question of where to place intervention by invitation in relation to the prohibition, as they simply assume that such an intervention can take place before moving on to other matters. It is thus essential to look beyond these assumptions and to properly examine this issue to draw wellfounded conclusions on the relation between intervention by invitation and the prohibition of the use of force.
Thus far, it has been established that intervention by invitation is not caught by the prohibition, but it remains to be determined why, i.e. whether it follows either the first or the second approach. The second approach advances the idea that the intervention does not violate the territorial integrity or the political independence of the inviting state. As these two terms were not meant to restrict the scope of the article, only to clarify it, this approach does not follow the interpretation and original intention of the drafters of the Charter. 154 Moreover, it is at times argued that an intervention by invitation on the side of a government that is engaged in a fight against a liberation movement struggling for its right to self-determination could perhaps constitute a violation of both the political independence of that state and the right to self-determination. 155 This would not necessarily be the political independence of the government, but that of the liberation movement and thus the people. This situation would only arise, however, in the limited cases of decolonisation, racist regimes, and the Palestinian people. 156 At present, these situations are rare. Some authors take the right to self-determination a step further and equate a struggle for self-determination with any type of civil war situation. It is subsequently argued that an intervention by invitation in any civil war goes against the political independence of the state (or rather that of its people) and violates the right to self-determination. 157 Following this line of reasoning, the issue surrounding the right to self-determination is better addressed together with the matter of the legitimate authority entitled to issue the invitation for the intervention and/ or the question of whether an intervention by invitation is allowed during a civil war, which is also precisely how these authors deal with this issue. 158 These matters have been excluded from the scope of this article, as they deserve an extensive study, which is not possible here. Whether an intervention by invitation itself violates the right to self-determination or potentially other norms of international law should thus be addressed elsewhere. For now, following either the narrow or broad interpretation of the right to self-determination, the result would be that the prohibition of the use of force will be considered to have been violated. This is not in line with the majority opinion that asserts that the prohibition is not violated in the case of an intervention by invitation.
Hence, as the notions of political independence and territorial sovereignty were only meant to clarify the scope of the prohibition of the use of force, not to limit it, and as arguing for a violation of the prohibition following the right to self-determination does not adhere to the established conclusion that the prohibition is not violated by an intervention by invitation, the second approach can be disregarded as well.
That leaves the first approach. Here it is advocated that the prohibition of the use of force is not violated by an intervention by invitation as the invited state does not use the force in its international relations. In contrast with the second approach, this first approach is in fact supported by the preparatory works of the UN Charter. The phrase 'against the territorial integrity and political independence' was added to clarify the article, not to limit its scope. 159 By focussing on the first part of the article, which was always intended to be incorporated, the interpretation which also reflects the original intent of the drafters of the Charter has been followed. 160 156 Doswald-Beck (1986), p. 207. 157 Bannelier and Christakis (2013), pp. 860-863;Wippman (1996), p. 212;Perkins (1986), pp. 183-190. 158 De Wet (2015, p. 981; Bannelier and Christakis (2013), pp. 860-863;Wippman (1996), pp. 209-213;Perkins (1986), pp. 190-193. 159 Randelzhofer and Dörr (2012), pp. 215-216;Henderson (2018), p. 20. 160 Perhaps intent as discussed by Corten (2010) and Ruys (2014) finds its place within the requirement of using force within international relations. Perhaps an accidental border incursion does not constitute a violation of the prohibition as it is not intended to happen within the 'attacking' state's international relations, nor in those of the 'attacked' state. But such a discussion is beyond the scope of this article. See also , pp. 74-75. Additionally, this first approach follows the distinction made above between intervention by invitation and collective self-defence. 161 As the former takes place solely within the inviting state's territory, self-defence does not need to be relied upon. The latter notion only becomes relevant when the attacked state wants to counterattack the attacking state on that latter state's territory (state X retaliating against state Y on Y's territory, with assistance from state Z). The notion of self-defence is not relevant when the force is used within a state's own territory (state A striking back against state B or against a non-state actor on A's own territory, with assistance from state C).
The conclusion is consequently that the force utilised within intervention by invitation is not used in international relations, precisely because the invitation has been issued. 162 Consent is thus built into the primary norm. It does not operate as a circumstance precluding wrongfulness, but it is incorporated in the phrasing of 'in their international relations'. Hence, the prohibition of the use of force does not apply and this type of the use of force is consequently allowed. 163

Conclusion
This article has focussed on those aspects of the concept of intervention by invitation that are often disregarded. In fact, the very name of the concept itself has long been taken for granted. As demonstrated here, a more suitable name would be use of force by invitation, as it is not merely an intervention which takes place, but a military one, which by its forceful nature must be classified as a use of force. The latter is a related yet separate concept under international law to which different rules apply. Moreover, even though intervention by invitation is traditionally understood to mean using force against a non-state actor, the term use of force by invitation can be used as a general term for any use of force by an invited state on the territory of the inviting state, irrespective of whether the force is used against a state or non-state actor. Other than the need for an armed attack, this is precisely where the difference lies with collective self-defence. A self-defence action takes place outside the attacked state's territory, while a use of force by invitation is limited to the territory of the attacked state.
As for the invitation part, this needs to come from the state itself, not from a nonstate actor. It must come from the highest available governmental authority, it must be provided free from pressure, and it must be issued prior to the use of force. The ensuing use of force must remain within the limits of the invitation.
The analysis subsequently determined how the use of force by invitation relates to the prohibition of the use of force as contained in Article 2(4) UN Charter. Four approaches were considered: intervention by invitation falls outside the scope of the prohibition, either because the force is not used in international relations or because it does not go against the territorial integrity or political independence of the inviting state; it is caught by the prohibition, but it constitutes an exception; or the wrongfulness is precluded by consent as a circumstance precluding wrongfulness. The first approach is favoured here, as using force upon the invitation of a state does not affect that state's international relations. Consent is built into the primary norm. This approach follows both the distinction that is made between the use of force by invitation and collective self-defence, and the original intentions of the drafters of the UN Charter. Since the second approach does not do so, it is thus unsuitable. The third approach is not appropriate either, as a customary exception to Article 2(4) is impossible due to Article 103 UN Charter. Even though many similarities can be identified between the notions of consent and invitation, it is not legally correct to speak of consent to the use of force. Consent is not a circumstance precluding wrongfulness proper, especially not for the use of force, as it is built into the primary rule. Article 20 ARSIWA is therefore redundant. To sum up, intervention by invitation, or rather the use of force by invitation, constitutes a legal use of force within the inviting state's territory as it is not caught by the prohibition of the use of force. In other words, a use of force by invitation would constitute an illegal use of force had the invitation not been given.
This article has demonstrated that the need still exists for more discussion and further investigation into the most basic features of jus ad bellum as its features are used without the exact definitions having been determined. That the concept of intervention by invitation is known by this term and not the more appropriate use of force by invitation is only the beginning. Indeed, when discussing intervention by invitation, the force will always be with you.