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 in areas that are usually less studied, such as those that use emerging technology or take place in newer military domains like outer space. In doing so, Erin Pobjie explains the emergence of the customary prohibition 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 students, legal scholars, and practitioners.