This conclusion sets out a definitional framework for a prohibited ‘use of force’ under article 2(4) of the UN Charter according to the type theory developed in this monograph. It sets out the contextual elements of that provision and the elements of a ‘use of force’ that are identified and discussed in greater detail in earlier parts of the monograph. Finally, it offers some reflections on the legal nature of this framework and its potential as a tool for scholars and practitioners to assess whether forcible incidents meet the threshold of a prohibited ‘use of force’ between States under international law.

This introduction highlights the prevailing uncertainty regarding the meaning of a ‘use of force’ under article 2(4) of the UN Charter and customary international law. It sets out the key research questions that this monograph addresses regarding the meaning of ‘use of force’ under jus ad bellum, including if ‘force’ means physical/armed force only and whether kinetic means or the use of particular weapons required, if a (potential) physical effect is required and the required nature of such effects, if there is a de minimis gravity threshold, and if a coercive or hostile intent is required. It also explains why the definition of prohibited force matters and its consequences under international law, including with respect to the gap between ‘use of force’ under article 2(4) and ‘armed attack’ under article 51 as well as the rise of grey zone operations. Finally, it sets out the aims and contributions of this monograph and an outline of its structure.