The Meaning of Prohibited ‘Use of Force’ in International Law

Abstract

Despite its central importance in the international legal order, there remains genuine uncertainty among States, scholars and jurists about the meaning of a prohibited ‘use of force’ under article 2(4) of the United Nations Charter and customary international law. This ambiguity is exploited by States and undermines compliance with the norm. This chapter accordingly presents and applies an original framework – type theory – to identify prohibited ‘uses of force’ between States, focusing on the meaning of ‘use of force’ as well as the contextual requirement of ‘international relations’. It argues that a prohibited ‘use of force’ between States is characterised by a basket of elements, not all of which must necessarily be present in order for an act to meet the definition. Instead, these elements – including certain types of effects, gravity and intention – are identified and weighed to determine whether the threshold of the definition of a ‘use of force’ is met. The theory of ‘type’ is firstly set out before explaining how it applies to the prohibition of the use of force, with illustrative examples from State practice, including the attempted killings of George Bush and Sergei Skripal and instances of excessive or unlawful maritime law enforcement. Finally, this chapter applies the type theory framework to a potential ‘use of force’ in an emerging military domain – the testing of anti-satellite (ASAT) weapons in outer space – to illustrate how it can be applied in novel contexts. In applying the type theory framework to these concrete case studies, this chapter draws definitive conclusions regarding the definition of a ‘use of force’ and demonstrates the usefulness of this theory as a tool for legal scholars and practitioners.

Keywords: Article 2(4), UN Charter, treaty interpretation, aggression, jus ad bellum, international relations, space security