This chapter presents and applies an original framework – type theory – to identify a prohibited ‘use of force’ between States under article 2(4) of the UN Charter, focusing on the meaning of ‘use of force’ and the contextual element of ‘international relations’. The theory of ‘type’ is firstly set out before explaining how it applies to the prohibition of the use of force. This chapter argues that a prohibited ‘use of force’ between States is characterised by a basket of elements, not all of which must be present for an act to meet the definition. Instead, these elements – including certain effects, gravity and intention – are identified and weighed to determine whether the definition of a ‘use of force’ is met. Illustrative examples are given from State practice including targeted killing and excessive or unlawful maritime law enforcement. Finally, this chapter applies the type theory framework to the testing of anti-satellite (ASAT) weapons in outer space to show how it can be applied in novel contexts. Through these case studies, this chapter draws definitive conclusions regarding the definition of a ‘use of force’ and demonstrates how to apply this framework in practice.

Humanity’s ascent into space began in 1929 when the German Army tested its first rocket, the A-1. But while militaries have always accounted for a large portion of human space activity, their use of the space environment has been constrained by a mutual self-interest in preserving access to it for communications, navigation, reconnaissance, weather forecasting, arms control verification and early warning. In 1962, the ‘Starfish Prime’ nuclear test demonstrated that nuclear explosions in space pose a major and indiscriminate threat to satellites. This prompted the United States and the Soviet Union to negotiate the 1963 Limited Test Ban Treaty, which prohibits nuclear tests in space. This chapter addresses such tensions between the expansion of military capabilities in space and the need to keep space free of direct conflict. The chapter highlights the growing need for a treaty to ban the testing of ‘kinetic’ anti-satellite weapons, i.e. weapons that rely on violent impacts to destroy a satellite and thus create space debris. Although Russia tested such a weapon in November 2021, the very next month the United Nations General Assembly created an ‘Open Ended Working Group on Reducing Space Threats through Norms, Rules and Principles of Responsible Behaviours’. The chapter concludes with an examination of the potentially destabilising effects of an imminent extension of military activities to cis-lunar space, the region between Earth and the Moon, including special Moon–Earth orbits.