Prohibited 'use of force' under article 2(4) of the UN Charter and customary international law has until now not been clearly defined, despite its central importance in the international legal order and for international peace and security. This book accordingly offers an original framework to identify prohibited uses of force, including those that use emerging technology or take place in newer military domains such as outer space. In doing so, Erin Pobjie explains the emergence of the customary prohibition of the use of force and its relationship with article 2(4) and identifies the elements of a prohibited 'use of force'. In a major contribution to the scholarship, the book proposes a framework that defines a 'use of force' in international law and applies this framework to illustrative case studies to demonstrate its usefulness as a tool for legal scholars, practitioners and students. This title is also available as Open Access on Cambridge Core.