Use of Force

Author: Marko Svicevic

This is a text created by OpenRewi. We are dedicated to creating Open Educational Resources. Follow us on Twitter or sign up for our newsletter.

Comments, critique or questions regarding parts of this chapter?

Use our texts in accordance with our Instructions for Reuse.

Preliminary Remarks

The official first edition of this chapter has been published by Routledge. It can be purchased as a printed book or downloaded. This Wikibook serves as an open access backup of the official version. It should be noted that some chapters have not yet been adapted to match the official version. For authoritative reading, we therefore refer you to the version published by Routledge.

For each chapter, interactive exercises have been created which can be accessed on our website. Simply follow this link or scan the QR code to access the exercises.

Required knowledge: History of International Law, Sources of International Law, International Organizations

Learning objectives: Understanding XY.
Understand the historical development, scope and extent of the prohibition of the use and threat of force
Identify and explain exceptions to the prohibition both within and outside of the UN Charter
Examine centralised and decentralised collective security (UN Security Council authorisation) as an exception to the prohibition

A. Introduction

One of the most important, and consequently, most controversial subjects in public international law, is the use of force.^[1] The use of force predominantly refers to military force; that is, where one or more State or international organisation uses military force against another State(s) or international organisation(s). Colloquially, the law on the use of force refers to a State's permissibility to go to war (also known as ‘armed conflict’ or ‘armed force’), and the laws governing the use of force are those which regulate when and under what conditions a State may (legally) go to war.^[2] The Latin term ‘jus ad bellum’ is also frequently used to refer to the use of force; its literal interpretation being ‘right to war.’

It is first and foremost important to note that the rules governing the use of force (jus ad bellum) predominantly regulate States' behaviour prior to their engagement in war as well as the decision of going to war. In contrast, the laws of war (jus in bello) regulate the permissibility of a State's conduct during war time.^[3]

Crucial to the rules on the use of force are the scope and extent to which it is prohibited. The fact that a State has, under certain circumstances, a right to resort to the use of force presupposes that the use of force itself is, in one way or another, prohibited. Although this has not always been the case, the adoption of the UN Charter in 1945 saw the prohibition codified in Article 2(4).^[4] At the same time, the UN Charter also established and re-affirmed certain exceptions to the prohibition. These include the inherent right of self-defence in Article 51, and in cases where the United Nations Security Council (UN Security Council) authorises the use of force by States or regional arrangements or agencies for the maintenance of international peace and security.^[5] The system of collective security put in place following the end of the second World War and the creation of the UN remains largely intact today. This system of collective security therefore not only prohibits the use of force, but also regulates it.

B. Historical Assessment of the Use of Force in International Law

I. The Just War doctrine

It may be surprising to many new scholars of international law that war was not always prohibited. Morevoer, prior to the comprehensive prohibition of the use of armed force in the 20th century, states, empires and kingdoms regularly claimed a right to wage war. Such claims were usually based on religious or moral grounds; a right to wage war to restore the peace or punish a an enemy. As wars progressed, and Christianity expanded across Europe, a fundamental issue arose when two Christian states went to war with each other. On this basis, Thomas Aquinas formulated the most comprehensive work on what is considered the 'just war theory'. In principle, the just war theory puts forward a moral or ethical reasoning behind why and how states go to war. Acquinas detailed three conditions for waging a just war:

War by the sovereign: only a sovereign or sovereign authority had the right to declare and subsequently wage a war. Private persons therefore had no right of their own to engage in war.
Just cause: waging war must have been for a 'just' cause. In other words, going to war could have been undertaken in response to an grave injustice or crime which was committed by the other party.
Just intention: the waging of the war must have been done with a 'just' intention by the sovereign, such as punishing a wrondoer, or bringing about a 'just' peace.

On this basis, the just war theory advocated for a moral basis to wage war. War fought for imperial reasons, self-interest, or the acquisition of territory could not reasonably fit into the just war theory. Of course, the very idea of a 'just' war was never as clear as imagined since it often involved at a matter of perspective and discretion. For obvious reasons, any number of wars for any number of purposes could be waged depending on ones construction of what is 'just'.^[6] At the same time, it should be mentioned that the 'just war' theory continues to face significant criticism. Because of its very construction, and in many ways, ongoing execution, just war theory has been perceived, and rightfully so, as an enalber wars in support of colonialism and the sub-judication of non-Western people's, drawing (arbitrary, western based) distinctions between civlised and uncilvised, and just and unjust.^[7]

II. From jus ad bellum to jus contra bello

Although the just war theory held for some four hundred years, the Peace Treaties of Westphalia of 1648 heralded a new right to wage war.^[8] State sovereignty, seen as absolute and above all else, considered the right to wage war as an extension of such sovereignty. More often than not, therefore, the right to wage war not only materialised, but was enshrined in states' national policies.^[9]

Despite attempts in particular during the end of the 19th and beginning of the 20th centuries to curtail means and methods of war, the most serious attempt at regulating the resort to war only came in 1919 after the end of World War I.^[10] The Treaty of Verailles, signed on 28 June 1919, declared an end to World War I and set in place the Covenant of the League of Nations. The League was meant to serve as an intergovernmental organisation of states which would seek to prevent war, increase international cooperation, accept not to resort to war, place limits on waging war including disarmament, submit disputes for arbitration and establish a Permanent Court of International Justice.^[11] Despite several commitments under the Covenant, it was only in 1928 with the adoption of the Kellogg-Briand Pact on 27 August 1928 in which the first multilateral international commitment to renounce war was made. Surprisingly, the Pact consists only of three provisions. Article I expresses the parties' condemnation of recourse to war to international issues and renouncement of war as an instrument of national policy.^[12] Article II declares the parties' intention to resolve their disputes by pacific means, and Article III concerns the treaty's ratification and entering into force.

Despite the Kellogg-BriandtPact's revolutionary renunciation of war in international relations, its success was limited. A decade later, the Pact's provisions were little more than empty promises, and the commencement of World War II laid bare its inability to suppress armed conflict. The comprehensive prohibition on the use or threat of force therefore only came into being following the end of World War II and the adoption of the UN Charter, together with its pivotal Article 2(4).

Advanced: The limitations of the prohibition of the use of force in the interwar instruments

Although the development of the principle prohibiting the use of force was first taken under the Kellogg-Briandt Pact it was ultimately unable to prevent the occurrence of the second World War.

Nevertheless, some strengths of the Pact included:

Its innovativeness as the the first attempt at prohibiting the resort to war in international law
Prior to this, waging war was not per se prohibited
It is widely considered the predecessor to the prohibition of the use of force found in Article 2(4) of the UN Charter
The Pact was signed by over 50 states, meaning that the majority of established States at the time had agreed/consented to the Pact’s provisions

Nevertheless, the Pact had a number of major weakenesses, chief among them included:

The lack of enforcement mechanisms (i.e., no penalties or sanctions for those who violate the Pact)
The use of the term war was very narrow; resort to war by reference to other means meant that States resorted to war without calling it war – the prohibition on ‘war’ was therefore a major weakness
The Pact was ultimately unable to prevent the outbreak of World War II

C. The Prohibition of the Threat or Use of Force

I. Scope of prohibition under Article 2(4) of the UN Charter

One of the cornerstones of the UN Charter system of collective security involves the prohibition of the use of force. Article 2(4) of the UN Charter effectively prohibits States from going to war with one another. Article 2(4) reads:^[13]

All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State, or in any other manner inconsistent with the Purposes of the United Nations.

From the wording of Article 2(4), it is clear that both threats of force and the actual use of force are prohibited. In addition, it is worth noting that the prohibition in Article 2(4) is framed within the context of States 'international relations.' The threat or use of force may therefore not be employed between States in their engagements with one another. Internal use of force by a State, such as police action, or the suppression of seccessionst or rebel movements, falls outside the scope of Article 2(4). The threat or use of force may therefore not be directed against another States' territorial integrity or political independence, or, in any manner inconsistent with the Purposes of the United Nations found in Article 1 of the UN Charter.^[14] While there are some who argue that the threat or use of force which is not directed against the political independence, territorial integrity or the Purposes of the United Nations would not violate the prohibition, the predominant interpretation of Article 2(4) is that it is a blanket ban against the use of force.^[15]

One question which arises with the interpretation of the prohibition is what exaclty is meant by 'force'. First and foremost, from the drafting history of the UN Charter and considering the broader law on the use force, the term 'force' refers to military or armed force. Pressue directed by means of political or economic coercion is not regulated by the law on the use of force.^[16] It is true that even today there is much debate as to what consitutes 'force': is only the use of conventional weapons required?; does one focus on the effects of the act rather than the instrument involved?; and, do cyberattacks qualify as uses of force?^[17] Albeit difficult to determine precisely, there is sufficient State and organisational practice, as well as ICJ jurisprudence, to at the least point out instances which clearly are uses of force:

the use of one State's military to target another State (the use of conventional armed forces for purposes of invasion, occupation or annexation)^[18]
acts of aggression (see further below), which may include bombardment, targetting of military forces, and annexation
supporting rebel movements
targeted killings (frequently undertaken with the use of drones in another State without that State's consent)^[19]
cyberattacks which cause have a scale and effect comparabel to kinetic uses of force (physical damage of kinetic effect)^[20]

Despite certain situations that clearly represent 'force', there remain a number of circumstances which remain uncertain:

do 'minimal' uses of force qualify as a use of force, and is there a gravity threshold: would the firing of one bullet across State borders qualify?^[21]
mistake or error: does the law on the use of force require intention? Would an accidental missile strike from one State into another qualify?^[22]
non-conventional attempted or targetted killings: does the poisoning of an individual by a prohibited substance in another State by an 'aggressor' State qualify as a use of force?

What is however far clearer are those circumstances in the law on the use of force which are categorised as 'acts of aggression'.

II. Threats of force

In comparison to actual uses of force, literature and practice pertaining to threats of force remains far less prominent.^[23] Generally, threats of force are those actions which fall short of actual use of force. In practice, a number of different actions meet this criteria. For example, one of the clearest manifestations includes an ultimatum, whereby a State is given a choice to comply with certain requests or demands, failing which, it will face the use of force against it.^[24] Other forms in which a threat of force manifests include open verbal communication by one State against another threatening force, as well as demonstrations of force - such as military buildups or exercises. It is generally understood however that threats of force are unlawful if the envisaged actual use of force that may follow from the threat is also unlawful.^[25]

In addition to their prohibition under the UN Charter, the use and threat of force bears consequences under other areas of international law too. Article 52 of the Vienna Convention on the Law of Treaties states that a treaty is void if its conclusion has been procured by the threat or use of force in violation of the principles of international law embodied in the UN Charter.^[26] Consequently, the conclusion of any treaty resulting from a threat or use of force will be void. The use and threat of force may also result in state responsibility, giving rise to reparations. Any unlawful use of force, considered an internationally wrongful act, therefore gives rise to state responsibility.^[27] Further, there are obligations on States not to recognise the lawfulness of a situation caused by the breach of peremporty norms; such a violation of the prohibition of the use of force for territorial acquisition.^[28] Moreover, under international criminal law, persons involved in the commission of acts during a violation of Article 2(4) give rise to individual criminal responsibility.^[29] Acts and conduct which therefore breach Article 2(4) are in no way isolated and have numerous consequences from other areas of international law. In other words, a violation of Article 2(4) never stands alone, always being too a violation of other principles and rules of international law.

III. Regional and sub-regional organisations incorporating the prohibition

The prohibition of the use of force in Article 2(4) of the UN Charter is similarly found in numerous regional and sub-regional instruments. Such prohibition is therefore reflected in regional treaty law and the constituent insturments of a number of international and regional organisations. To name but a few of these:

Articles 19, 21, and 22 of the Charter of the Organisation of American States (OAS), for example, contains a number of provision curtaining resort to force which also provide for the peaceful settlement of disputes.

Article 4(f) of the Constitutive Act of the African Union (AU), successor to the Organisation of African Unity (OAU), contains a similar provision to Article 2(4) of the UN Charter.
Article 1 of the Economic Community of West African States (ECOWAS) Protocol on Non-Aggression sets out that member states in their international relations refrain from the threat or use of force or aggression.^[30]
Articles 3 and 4 of the International Conference on the Great Lakes Region (ICGLR) Protocol on Non-Aggression and Mutual Defence similarly requires member states to renounce the threat or use of force and includes a comprehensive prohibition on the threat or use of force.^[31]

Although the prohibition does not appear explicitly in several other regional-organisations; other principles on the jus ad bellum are prominently featured, such as the prevention and resolution of inter-state conflict, good neighbourliness, the resolution of disputes by peaceful means, and the use of force only as a last resort.

D. Exceptions to the Prohibition

I. Exceptions within the UN Charter system of collective security

1. Centralised collective security: UN Security Council Chapter VII authorisation

Within the provisions of the UN Charter, there exist two principal exceptions to the prohibition of the use of force; that is, two circumstances under which the use of military force is permitted. The first of these, which is discussed here and in sub-section 2 directly hereunder, concerns situations where the UN Security Council authorises the use of force by States or regional organisations. The second exception found within the UN Charter, discussed further blow, concerns individual or collective self-defence.

The UN Charter system of collective security provides that force in not only prohibited (Article 2(4), but that it is also regulated in so far as the UN Security Council may authorise the use of force (Chapters VII and VIII). The basis for the UN Security Council's competence is found in Article 24(1), which provides that '[i]n order to ensure prompt and effective action by the [UN], its Members confer on the Security Council primary responsibility for the maintenance of international peace and security, and agree that in carrying out its duties under this responsibility the Security Council acts on their behalf.' In principle therefore, member states, upon ratifying the UN Charter, agreed that the UN Security Council could act on their behalf.

Indeed, the UN Security Council has authorised the use of force on a number of occasions. The most notable of these include authorisation to member states during the Korean War (1950) and the Iraqi invasion of Kuwait (1990).^[32] An examination of UN Security Council practice in this regard shows a number of characteristics when it comes to authorising force. These characteristics largely conform to the provisions of the UN Charter and are supported broadly in scholarship:

Before authorising the use of force, the UN Security Council must first make an Article 39 determination on a given situation. In other words, the UN Security Council determines the existence of a threat to international peace and security.^[33] From its practice, the UN Security Council's wording of this determination varies: it has however consistently determined situations as (a) a threat to international peace and security, (b) a threat to peace and security in the region, (c) a threat to the peace, or (c) a breach of the peace.^[34]

Once the UN Security Council has made an Article 39 determination, it has a broad range of options available to it in order to restore international peace and security. These include both non-coercive (non-military) and coercive (military measures). The UN Security Council can, by way of non-coercive measures, call on the parties to a dispute to comply with provisional measures such as a ceasefire agreement.^[35] The UN Security Council may go a step further and impose additional measures, such as placing sanctions on individuals and organisations or subjecting them to travel bans and freezing their assets.^[36]

When non-coercive measures prove insufficient, the UN Security Council may decide on military measures under Article 42 of the UN Charter. Article 42 reads:

Should the Security Council consider that measures provided for in Article 41 would be inadequate or have proved to be inadequate, it may take such action by air, sea, or land forces as may be necessary to maintain or restore international peace and security. Such action may include demonstrations, blockade, and other operations by air, sea, or land forces of Members of the United Nations.

In practice, the UN Security Council has however never invoked Article 42 in its resolutions authorising the use of force. Neither does it employ the language of Article 42. Instead, it has consistently used the terms 'all necessary means' when signalling its authorisation for the use of force. Moreover, in practice, since the UN has no standing army,^[37] the UN Security Council authorises these military measures to UN Member States; in so doing, these Member States therefore act on behalf of the UN Security Council when employing force.^[38] This system of authorisation of the use of force has come to be known as delegated enforcement action.^[39] Since it first employed such a technique during the Korean War in 1950, the UN Security Council continues this practice today, having authorised the use of force by Member States on over a dozen occasions.^[40]

Finally, when authorising the use of force by Member States, the UN Security Council has adopted a relatively consistent practice of invoking Chapter VII in those resolutions authorising the use of such force. Not only is this understandable in so far as the UN Security Council is clearly acting under Chapter VII powers, but resolutions which invoke that chapter have consistently been interpreted with a greater gravity and sense of bindingness. To this end, most resolutions authorising the use of force contain, often in the final preambular paragraph, the UN Security Council's expression that it is 'acting under Chapter VII of the Charter of the United Nations.'^[41]

2. Decentralised collective security under Chapter VIII

As much as the UN Security Council may authorise the use of force by States, it may also, and regularly does, authorise the use of force by regional organisations. This system of authorisation is sometimes known as decentralised collective security in so far it is regional organisations which act on behalf of the UN Security Council.

Article 52 of the UN Charter recognises the important role that regional organisations could play in matters of peace and security. As such, it expressly provides that nothing in the UN Charter precludes 'the existence of [regional organisations] for dealing with such matters relating to the maintenance of international peace and security...'^[42] Regional organisations are of course obliged to make use of pacific settlement of disputes prior to their referral to the UN Security Council.^[43]

It is worth noting that the UN Charter consistently refers, not to 'regional organisations', but rather, to 'regional arrangements or agencies.' Nonetheless, this reference clearly speaks of regional organisations to the extent that such organisations are involved in matters of peace and security within a specific geographical region of operation and over such region as where its members are situated.^[44] Examples of regional organisations of such a nature include the African Union and its Regional Economic Communities, the European Union, and the Organisation of American States.^[45] One organisation which has evaded being considered as a 'regional arrangement or reginal agency' is the North Atlantic Treaty Organisation (NATO) - which resembles more of a collective defence alliance. Despite this, scholars have convincingly argued that it is not necessarily the nature of the organisation which would determine whether UN Security Council authorisation of the use of force is required, but rather, the nature of the action in question.^[46] In other words, an organisation which acts in collective self-defence, that is coming to aid of another State which has been attacked, need not obtain UN Security Council authorisation. An organisation which resorts to force for other purposes however, such a maintaining peace and security, will require UN Security Council authorisation.

When peaceful means of resolving a dispute fail, the UN Security Council may authorise a regional organisation to use force under Article 53(1) of the UN Charter.

The wording of Article 53(1) has not been spared of debate surrounding its interpretation. Although the UN Security Council has authorised the use of force by regional organisations on numerous occasions,^[47] there remains some uncertainty among scholars as to whether regional organisations may use force without the approval of the UN Security Council.^[48] A further question which arises is whether UN Security Council for the use of force needs to be granted explicitly and prior to the actual use of force by the organisation in question. This debate becomes particularly relevant when considering that some regional organisations and defence alliances, have either developed their own treaty law seemingly allowing them to use force unilaterally, or have resorted to the actual use of force without UN Security Council approval. Examples often cited include the ECOWAS interventions in Sierra Leone and Liberia in the 1990s, and the NATO bombardment of Yugoslavia in 1999.^[49] Where the development of an organisation's institutional treaty law arises, one of the most widely cited examples is that of Article 4(h) of the Constitutive Act of the African Union. Article 4(h) allows the AU to intervene into its Member States when grave circumstances occur, which include war crimes, genocide, and crimes against humanity. The fact that the Constitutive Act makes no mention of whether the AU requires UN Security Council approval rightfully invokes uncertainty as to whether UN Security Council involvement is at all required.^[50]

Advanced: The United Nations General Assembly and the Uniting for Peace Resolution

Following from the Korean War in the 1950s and the paralysis of the permanent members of the UN Security Council at the time, the UN General Assembly adopted Resolution 377A on 3 November 1950. The resolution came to be known as the 'Uniting for Peace' resolution - aimed principally to allow the General Assembly to consider and take actions on matters of peace and security when the UN Security Council itself failed to discharge its responsibilities under the UN Charter. One of the most important provisions of the resolution is found in the first operative paragraph, which reads: '1. Resolves that if the Security Council, because of lack of unanimity of the permanent members, fails to exercise its primary responsibility for the maintenance of international peace and security in any case where there appears to be a threat to the peace, breach of the peace, or act of aggression, the General Assembly shall consider the matter immediately with a view to making appropriate recommendations to Members for collective measures, including in the case of a breach of the peace or act of aggression the use of armed force when necessary, to maintain or restore international peace and security. If not in session at the time, the General Assembly may meet in emergency special session within twentyfour hours of the request therefor. Such emergency special session shall be called if requested by the Security Council on the vote of any seven members, or by a majority of the Members of the United Nations; '

Despite the resolution's strong wording, there remains debate today as to whether the General Assembly has the competence to consider and take actions with respect to breaches of the peace, acts of aggression and threats to peace and security.

It is worth noting however that to date, the resolution has been invoked 13 times (eight times by the Security Council and five times by the General Assembly). The most recent invokation of Uniting for Peace related to the Russian invasion of Ukraine on 24 February 2022 under UN Security Council Resolution 2623 (2022).

3. Individual and collective self-defence

a. Article 51 of the UN Charter

One of the most relied upon exceptions to the prohibition of the use of force involves the right of individual and collective self-defence. Although this right has been codified in Article 51 of the UN Charter, it is well established that the right is inherent; that it has been at the disposal of States even before the UN Charter was adopted. The International Court of Justice confirmed this position in its Nicaragua judgment when it stated that the right of self-defence is a 'natural' or 'inherent' right and its inclusion in the UN Charter 'does not go on to regulate directly all aspects of its content.'^[51] The right of self-defence means that all States may use military force when they are attacked by other States, or where such an attack is imminent. Article 51 of the UN Charter recognises this right in the following words:^[52]

While States have resorted to this right historically on many occasions, the contemporary understanding of the right features a number of characteristics. These may either be specific requirements before a State may rely on self-defence, or they may be limitations to the right.

b. The requirment of an 'armed attack'

In order for a State to rely on self-defence, there must first exist an armed attack. That is, before a State may use armed force in self-defence, it must have been attacked (what in international law is referred to as an 'armed attack'). Armed attack is however difficult to define and has been a point of scrutiny and continuesto encounter divergent interpretations. Nonetheless, an armed attack involves military force by one State against another. Such an attack also needs to be of a certain gravity; therefore, not all uses of force are considered an armed attack. The ICJ in the Nicaragua case did give some insight into armed attack and its gravity. For example, an armed attack could be, not only a 'frontier incident', but a case where 'the sending by a State of armed bands to the territory of another State' and that such action was of a 'scale and effect' to be considered an armed attack.^[53] It is worth noting that the Court's position on outlining more grave and less grave incidents has come under considerable scrutiny over the years.^[54] In any case, what is clear is that not all unlawful uses of force are considered armed attacks and that armed attacks are narrower in scope.

c. Limitations to the (lawful) exercise of self-defence

As can be expected, the right to use force in self-defence is not unlimited. In this context, two limitations need discussion: necessity and proportionality. These limitations are not spelled out in Article 51 of the UN Charter but are derived from customary international law (as reaffirmed by the ICJ in the Nicaragua case^[55] and the Oil Platforms case).^[56]

The first limitation is necessity; that the use of force in self-defence is necessary. This entails for example, that only the resort to force could achieve a specific goal, such as countering an armed attack. By implication therefore, necessity of forceful measures means that other dispute resolution options were not available to State. It also means that that peaceful means of resolving a situation were unavailable to the State relying on self-defence.

The second limitation attached to the lawful exercise of self-defence is proportionality. This limitation regulates the gravity of the force used when a State repels an armed attack. The gravity of force should, in principle, be proportionate to counter or repel the armed attack, and only to counter the attack which has begun or is ongoing.^[57] The use of force in self-defence cannot, for example, both counter an armed attack and subsequently punish the attacking State or neutralise its armed forces entirely. Such counter measures would not only violate the proportionality requirement under self-defence but may also be a violation of the prohibition of the use of force.

d. The duration of the right of self-defence and the reporting requirement

While both necessity and proportionality serve as limitations to the right of self-defence under customary international law, Article 51 of the UN Charter imposes two additional criteria. The first of these criteria impacts the duration of validity of self-defence; how long a State may resort to use force in self-defence lawfully. While not spared from debate itself, the wording of Article 51 suggests that the use of force in self-defence cannot continue in perpetuity. States may resort to self-defence 'until the Security Council has taken measures necessary to maintain international peace and security.' Once the UN Security Council has taken such measures, a State cannot continue to rely on self-defence any longer. The UN Security Council may take any number of measures, and it may well be that it authorises the use of force, in order to restore international peace and security.^[58] That however does of course not mean that the UN Security Council needs to authorise self-defence; this option remaining at the disposal of States without UN Security Council approval.^[59]

The second criteria concerns a procedural requirement; when States resort to the use of armed force in self-defence, they are obliged to immediately inform the UN Security Council of this. In this way, the UN Security Council is kept abreast of latest developments and is able to assess situations as the primary organ responsible for international peace and security.^[60]

e. Anticipatory/pre-emptive self-defence and the ‘Caroline test’

One aspect of the right to self-defence which has received more than its fair share of debate concerns whether a State may use force in self-defence where an armed attack has not yet taken place but is imminent. This debate is certainly not new and is often considered against the Caroline test. The Caroline test, so called, arose from a dispute between America and Britain in the context of a rebellion in Canada.^[61] The Caroline was a steamboat which was transporting supplies to Canada, aiding the uprising there. The British set out to halt this, captured the boat, set it alight and sent it over the Niagara Falls. The British put forward that these actions amounted to self-defence. The Americans countered this explanation by putting forward the, now famously known, Caroline test. The Caroline test stipulates that a State claiming self-defence would need to show that the 'necessity of self-defense was instant, overwhelming, leaving no choice of means, and no moment of deliberation...'

This qualification of self-defence has lead to several arguments in favour of anticipatory or pre-emptive self-defence. The notion of anticipatory self-defence holds that a State may use armed force against another State even before an armed attack has taken place; such armed attack being however imminent.^[62] This line of thought has been furthered somewhat to include also 'preventive self-defence'; which postulates the use of force against another State even when an armed attack is not necessarily imminent.^[63]

Preemptive self-defence was used as one justification for the US invasion of Iraq in 2003. It was then argued by the US, together with Australia and the United Kingdom, that Iraq was attempting to acquire weapons of mass destruction and that this threat was too grave to avoid counter action. No armed attack took place against either of the States involved. Since then, preemptive self-defence has been widely criticized for violating Article 51 of the UN Charter and arguments in favour of such a notion have significantly decreased.^[64]

e. Self-defence against non-state actors

Traditionally, principal actors in international law have always been States. This has been true both for general international law and for the use of force. In contemporary times however, States as principal actors in the international system finds some challenge. No where is this more clear than when considering self-defence against non-state actors. Whereas only States were previously able to declare and wage war, there exists today (as has been the case for some time now), non-state actors which are sufficiently sophisticated and organised to wage both terrorist attacks and large scale conflicts against States. This is especially true of terrorist organisations such as Al Qaeda, Al Shabab, Boko Haram and the Islamic State. The terrorist attacks of 11 September 2001 against the United States and its corresponding 'war on terror' marked a turning point in the way international law, and more specifically, the use of force, was viewed.^[65]

The question posited ever since is whether States could rely on self-defence when using force against non-state actors. In turn, a further question which arises is whether non-state actors could at all mount an 'armed attack', to which a State could respond through the use of force in self-defence. Suffice it to say that both literature and State practice on this matter remains moot. Most recently, the UN Security Council held an Arria formula meeting (informal meetings convened by a member of the UN Security Council to hear views on a specific matter for which the UN Security Council has competence over) convened by Mexico to provide States with an opportunity to make submissions on this matter.^[66] Submissions by States during this meeting broadly confirmed a divergent approach. While some States took the view that the use of force against non-state actors was acceptable even in the territory of third States, other States viewed certain mandatory conditions which are required, such as obtaining the consent of the host State before using force.^[67]

II. Exceptions beyond the UN Charter

1. Military assistance on request

The sending of military forces by one State to the territory of another remains a surprisingly frequent occurrence. These actions are ordinarily justified in that the State sending the armed forces does so on the basis of a request by the inviting State. International law in this regard recognises the principle of 'military assistance on request', often also termed as 'intervention by invitation'.

In other words, State A may request, for one or another reason, that State B send its armed forces to State A's territory. Such a course of action would not violate the Article 2(4) prohibition of the use of force since the armed deployment is undertaken with the consent of the 'host state'.^[68] Despite its lawful nature, there are however a number of conditions which need to be met for such a course of action to be legal under international law.^[69]

First, the consent expressed by the host state must be given validly. The State requesting the military assistance must therefore have given its consent freely, without coercion.^[70]

Second, the authority of the host state giving the consent must be appropriate; that the entity consenting to or requesting military assistance in fact has the power to give such consent on make such request. Ordinarily, the authority entitled to request military assistance is the de jure government of a particular State (the internationally recognised government). In turn, the entities which are entitled to make such a request are those acting on behalf of the State (an organ of State), such as the head of state or head of government.^[71]

Finally, a State requesting military assistance may not receive it when it is under a state of civil war (the rule of non-intervention in civil wars).^[72] The general nature of this prohibition serves as a limitation to military assistance on request and regards that all people's have a right to self-determination and the right to choose the political, economic and social characteristics of the State, without external interference from, for example, military forces of another State.^[73] In so far as withdrawal of military forces concerns, it is worth noting that the ICJ in its DRC v Uganda case underlined that no particular formalities are required when a State wishes to withdraw its consent.^[74]

2. The use of force for humanitarian purposes: Humanitarian intervention and the Responsibility to Protect

One of the more controversial uses of force in international law concerns coercive measures taken in a State for humanitarian purposes, with neither its consent nor the authorisation of the UN Security Council. One such example includes humanitarian intervention. Such intervention is usually argued for the purpose of 'preventing or putting to a halt gross and massive violations of human rights or international humanitarian law.'^[75] This kind of intervention makes clear that it is taken against a State without its consent, and specifically for the purposes mentioned. Second, humanitarian intervention may be undertaken with or without the authorisation of the UN Security Council.

When humanitarian intervention is undertaken with the authorisation of the UN Security Council, it is followed by far less controversy. This is because the UN Security Council indeed has the competence to authorise the use of force against a state and for such purposes. With the end of the Cold War and particularly from the 1990s onward, the UN Security Council has on several occasions authorised the use of force by states for humanitarian purposes. Resolution 794 (1992) addressing the situation in Somalia, Resolution 929 (1994) on Rwanda, and Resolution 1080 (1996) on the Great Lakes Region, all represent to a greater or lesser extent examples where the UN Security Council has authorised military intervention for humanitarian reasons.^[76]

Unilateral humanitarian intervention on the other hand is far more controversial. It contemplates the decision of one or more states to use force in another state without its consent and without the authorisation of the UN Security Council. The most notable example of unilateral humanitarian intervention involves the albeit controversial decision of the North Atlantic Treaty Organisation's use of force against the Federal Republic of Yugoslavia in 1999,when it made use of military force to halt widespread crimes against humanity, war crimes and ethnic cleansing of Kosovar Albanians by Serbian military forces.^[77] Kosovo however is not a solitary example. The ECOWAS interventions in Liberia and Sierra Leone in the 1990s, and the US-led no-fly zone in Northern Iraq in 1991 are equally noteworthy in what has been argued as examples of humanitarian intervention.^[78]

On this basis, arguments both in favour of and against humanitarian intervention may be summarised as follows:

Arguments in favour of humanitarian intervention:

it provides a legal basis for the protection of civilians in situations where the UN Security Council is inactive or fails to discharge its responsibilities
it may ensure that desperately needed humanitarian aid reaches those affected
it rarely achieves lasting peace (States which have previously been the target of humanitarian intervention, whether authorised or not by the UN Security Council, do not seem to have achieved a durable and long lasting peace)

Arguments disfavouring humanitarian intervention:

there is no firm legal basis in international law for humanitarian intervention (it therefore remains controversial legally)
its application remains highly selective by States justifying their actions on this basis (some States are perceived as insignificant resulting in perceived indifference to specific situations)
politically, humanitarian intervention is opposed by a large majority of the third world (African States have expressed a preference for UN Security Council authorised actions, failing which, such action should be undertaken by regional organisations rather than Western States)^[79]

Beyond the concept of humanitarian intervention, there is also the development of the albeit distinct commitment referred to as Responsibility to protect (R2P). The idea of R2P arose from the work of the International Commission on Intervention and State Sovereignty which first postulated that although state sovereignty entails the right of each state to regulate domestic affairs, such a right was accompanied by the responsibility of each state to protect populations living within its borders.^[80] Where the state fails to protect its population, the responsibility to protect then rests with the international community. The principles behind R2P were formally endorsed by the UN General Assembly under its 2005 World Summit Outcome Document.^[81] In principle, R2P is based on three pillars: Pillar I: that each state carries with it the responsibility to protect its populations from genocide, war crimes, ethnic cleansing, and crimes against humanity, Pillar II: that states support each other in their responsibility to protect, and Pillar III: where a state fails to protect its population, that responsibility turns to the international community to ensure effective collective action.

Although both the idea of humanitarian intervention and its extension under R2P have been around for some time, its exception to the prohibition of the use of force in Article 2(4) remains controversial. Much of the controversy surrounding the concepts concerns the authority of States to intervene in each other's affairs and whether existing institutions such as the UN Security Council should in fact always authorise military force even in cases of humanitarian intervention. It therefore suffices to say that despite some practice and the ironing out of certain principles on the international level, the broader concept of humanitarian intervention can neither be considered a well-established exception to the prohibition nor accepted by the international community of States as such.

E. Summary

The use of force, its prohibition and exceptions to this prohibition have a long standing history in international law. While resort to armed force was not always regulated, nor was it always prohibited, the developments of the last century represent the greatest attempt to curtail armed force. Article 2(4) of the UN Charter and its prohibition of the threat or use of force remains a cornerstone not only of the UN Charter system of collective security but of the entire international legal order. Likewise, well established exceptions to the prohibition, most notably, self-defence under Article 51, and UN Security Council authorisation, are firmly established and widely accepted in practice. At the same time, the prohibition and corresponding exceptions are not without controversy. Emerging challenges, least of which include the terrorism and non-state actors, the continued proliferation of weapons of mass destruction, cybersecurity and cyberthreats, and abusive interpretations of self-defence, post significant challenges to the regulation of armed force. While the jus ad bellum has been resilient, the development of international law and of the use of force cannot be viewed in isolation. Its continued evolution will rest with its ability to meet the challenges of the 21^st century, ever present and expanding as they may be.

Further Sources

EJIL:Talk! category on the 'use of force' posts: https://www.ejiltalk.org/category/use-of-force/

Opinio Juris category on the 'use of force' posts: http://opiniojuris.org/category/topics/use-of-force/

United Nations, 'UN Web TV', UN Security Council: https://media.un.org/en/search/categories/meetings-events/security-council