States frequently make claims that they are using force in accordance with international law. For example, the United States and Israel have recently invoked an expansive understanding of the right of self-defence purportedly to justify their airstrikes against Iran in June 2025, as did Russia in relation to its 2022 invasion of Ukraine. The United Kingdom continues to invoke a supposed right of unilateral ‘humanitarian intervention’ as the legal basis for its uses of force against other states, most recently when conducting air strikes against Syria in 2018. Such claims are based on novel, and often controversial, analyses of the international law regulating the use of force (the jus ad bellum). Yet even superficial consideration of the rules that govern how international law is created and modified – rules to which these states profess to adhere – demonstrates that many of these novel claims by states are unsustainable. In this context, Dr. Johnston will set out her analysis of how new legal bases for the use of force by states may be created and how the existing bases for the lawful use of force by states under international law (the right of self-defence and Security Council authorisation) may be modified, if one applies the rules for the modification of international law rigorously. It will be argued that the structure of the international law on the use of force presents significant obstacles to the creation of new legal bases for the use of force (e.g. a new right of ‘humanitarian intervention’); but that there is potential for certain changes to the conditions for self-defence and Security Council authorisation to be achieved more easily.
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