In which situations may a State lawfully use force against another State? Is it possible that the law may change over time, so that what would previously be considered an unlawful use of force will come to be seen as lawful?
These questions were at the centre of the debate about the legality of the airstrikes against the Assad government in Syria, carried out by the US, UK and France on 13 April 2018. Yet they also go to the heart of the modern jus ad bellum, the rules of international law that govern the use of force by States in their international relations. My research looks at how the law in this area is structured and aims to identify the processes by which the jus ad bellum can change.
International law, as a decentralised system of States, has no central legislative body that enacts binding laws for the international community. States become subject to international legal obligations primarily by entering into treaties and through the emergence of rules of customary law as a result of the practice of States.
The adoption of the United Nations Charter in 1945 brought about a paradigm shift in how international law regulated war. From a sovereign right and a legitimate policy choice, albeit subject to increasing restriction over the first half of the twentieth century, the use of force by States became subject to a comprehensive prohibition, set out in Article 2(4) of the United Nations Charter. There are only two narrow exceptions to this prohibition, when force may lawfully be used: collective measures authorised by the Security Council under Chapter VII of the Charter, and the right of States to use necessary and proportionate force unilaterally in self-defence, if an armed attack occurs.
In addition to this treaty law rule in Article 2(4) which binds all UN member States, the prohibition on force also developed into a parallel rule of customary international law, binding on all States. It is also widely accepted that the prohibition on the use of force has the status of a “jus cogens” norm of international law, from which no derogation is permitted. Although the existence, definition and content of this category of international legal rules remains contested, typically jus cogens norms protect the fundamental values of the international community and include, for example the prohibitions on genocide and torture.
Thus, the prohibition on the use of force is expressed in three forms: the treaty provision in Article 2(4) of the Charter, a parallel rule of customary international law, and a jus cogens norm. To draw an imperfect analogy with domestic law, imagine the same prohibition of murder existing in a criminal statute, as a common law rule developed by the courts, and as a provision in the Constitution.
This account is necessarily a simplified one that skips over the many debates surrounding the definition of force, self-defence, and the functioning of the collective security system that have arisen since the Charter’s adoption, and some of which remain unresolved. However, fundamentally, this is the international legal framework that regulates the use of force by States today: a comprehensive prohibition on the use of force, with two narrow exceptions.
It is against this backdrop that the UK government’s claimed legal basis for the airstrikes on 13 April 2018 must be assessed. In a legal position published on 14 April, the UK government asserted that “the UK is permitted under international law, on an exceptional basis, to take measures in order to alleviate overwhelming humanitarian suffering.” This is almost certainly not correct under international law as it stands – as noted above, there are only two exceptions to the prohibition on the use of force, neither of which applies in this case. The UK position is inconsistent with the text of the Charter and very few States take the view that a legal right of humanitarian intervention currently exists, while many more have explicitly stated that they do not recognise the existence of any such doctrine.
Indeed, for the UK position to be correct, it would need to be shown that the legal framework set out above had evolved since 1945 so that a new legal basis for the unilateral use of force by states had come into existence. This could be, for example, through a change to the prohibition so that it no longer banned force used for humanitarian purposes, or creation of a third exception of humanitarian intervention. However, given the complex structure of the law in this area, any argument that the jus ad bellum has changed faces a number of obstacles.
First, as described above, the prohibition on the use of force exists in multiple legal rules. Therefore, even if a State could show that a new customary international law rule permitting humanitarian intervention had come into existence, any State claiming to rely on that right would still be in violation of its treaty obligation as a UN member not to breach the prohibition on the use of force in Article 2(4) of the Charter. This situation is further complicated by Article 103 of the UN Charter, which provides that the obligations of UN members under the Charter take precedence over their other obligations. Any State wishing to rely on a right of humanitarian intervention would therefore need to show not only that a new rule of customary law had emerged – which requires widespread and consistent practice by States accompanied by their belief that such a legal rule exists – but also that the UN Charter had been reinterpreted to allow for humanitarian intervention. Demonstrating such a reinterpretation, although possible in principle, is a difficult standard to meet: practice would need to establish the agreement of all UN members that the Charter should now be interpreted in this way.
A second, more difficult, obstacle is presented by the jus cogens status of the prohibition on the use of force. Jus cogens norms can only be modified by another norm of the same character, so it seems that any new legal basis permitting humanitarian intervention would also need to show that it had fulfilled the test for emergence of a new jus cogens norm: acceptance and recognition by the “international community as a whole” that the norm has such a status. This threshold is lower than that required to demonstrate reinterpretation of the UN Charter through subsequent practice, described above, but there are other characteristics of jus cogens norms that may complicate the process of change. Jus cogens norms invalidate contrary practice, depriving it of any legal effect it may have to bring about an evolution in customary law. The presence of the jus cogens norm therefore seems to put those arguing for a change to the prohibition of the use of force in an all-or-nothing position: either they show that the stringent requirements for changing a jus cogens norm have been met, or any practice short of that threshold will not only be in violation of the jus cogens prohibition, but also ineffective to bring about a change in the customary or treaty norms.
In sum, the existence of multiple legal rules prohibiting the use of force, and the characteristics of the Charter and the jus cogens norm that cause them to prevail over conflicting rules, mean that changing the prohibition on the use of force appears to be very difficult indeed. The UK legal position does not explain whether the purported new right of humanitarian intervention on which it relies is understood as a new rule of custom, a reinterpretation of the Charter, or a new jus cogens norm. As shown above, it appears that a plausible claim may need to demonstrate that it is all three, simultaneously.
There are good reasons why it should be difficult to change the prohibition on the use of force. Conceptual and methodological arguments about legal rules gloss over the death, suffering and displacement that is caused by armed conflict; historically, the impact on women has been particularly overlooked. The drafters of the Charter wanted to “save succeeding generations from the scourge of war” by creating a new international legal order where unilateral use of force by individual States became the rare exception and not the rule. Even if the prohibition on the use of force is not universally complied with (what law is?), the current legal framework requires States to justify any use of force in terms of its requirements or face international condemnation, which in itself has a restraining effect. Creating a new legal basis for States to use force – without collective authorisation – would upset the current balance, tilting it in favour of greater unilateralism. In addition, in the specific context of humanitarian intervention, any increased permissiveness is unlikely to benefit all States equally. States that already enjoy greater military and political power will be more able to take advantage of a new right to use force unilaterally. Less powerful States, on the other hand, will see their protection from foreign intervention diminished.
Nevertheless, the arguments remain finely balanced. While a legal right of humanitarian intervention does not exist in international law today, with the Security Council sliding back into gridlock and unable to address threats to peace and humanitarian disasters, international consensus may well shift to accept that intervention without Security Council authorisation is permissible in some circumstances. In such a case, it may be preferable for the international legal framework to be able to evolve to accommodate a new legal basis for the use of force, either within or in addition to the existing exceptions, rather than risk the prohibition being repeatedly violated or ignored, as appears to have been the case with the US, UK and French strikes last month. Developments such as the provision in the Constitutive Act of the African Union for a collective, regional, treaty-based right of humanitarian intervention could provide a less disruptive alternative to increased unilateral uses of force. Yet, as the law stands, this provision appears to be unlawful.
Clarifying the processes by which the rules of the jus ad bellum can change could help provide a shared basis for States and international lawyers to evaluate claims that the rules governing the law on the use of force have changed, and avoid a more unstable situation where States instead abandon or undermine the authority of the prohibition on the use of force or the Charter system. It is this question, of how the prohibition on the use of force and its exceptions can change, that my research will address.
 See, for example, Milena Sterio, Syria and the Limits of International Law, IntLawGrrls, 12 April 2018: https://ilg2.org/2018/04/12/syria-and-the-limits-of-international-law/
 Charter of the United Nations (1945): https://www.un.org/en/sections/un-charter/chapter-i/index.html
 As later confirmed by the International Court of Justice, Militarv and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America). Merits, Judgment, I.C.J. Reports 1986, p. 14, at para. 190.
 Syria action – UK government legal position, 14 April 2018: https://www.gov.uk/government/publications/syria-action-uk-government-legal-position/syria-action-uk-government-legal-position
 As noted by Professor Dapo Akande in his legal opinion of 16 April: Akande, The Legality of the UK’s Air Strikes on the Assad Government in Syria, 16 April 2018: https://www.scribd.com/document/376483861/Akande-Opinion-UK-Government-s-Legal-Position-on-Syria-Strike-April-2018
 Vienna Convention on the Law of Treaties (1969), Article 31(3)(b): http://legal.un.org/ilc/texts/instruments/english/conventions/1_1_1969.pdf
 VCLT, Article 53.
 Orakhelashvili, Changing Jus Cogens Through State Practice? The Case of the Prohibition of the Use of Force and its Exceptions, in Weller (ed.), The Oxford Handbook of the Use of Force in International Law (OUP 2015).
 Hilary Charlesworth, Christine Chinkin, Shelley Wright, Feminist Approaches to International Law, 85 American Journal of International Law, 613, 645 (1991); Christine Chinkin, A Gendered Perspective to the International Use of Force, 12 Australian Yearbook of International Law 279, 293 (1988-1989).
 Charter of the United Nations, Preamble.
 See Anne-Charlotte Martineau, Concerning Violence: A Post-Colonial Reading of the Debate on the Use of Force, Leiden Journal of International Law (2016), 29, pp. 95–112.
 Constitutive Act of the African Union (2000), Article 4(h): https://au.int/sites/default/files/treaties/7758-treaty-0021_-_constitutive_act_of_the_african_union_e.pdf