This chapter examines the meaning of a ‘use of force’ under article 2(4) of the UN Charter, focusing on its required means. It analyses whether ‘force’ in article 2(4) is restricted to particular means, namely, if it refers to physical/armed force only, if a weapon must be employed, what is considered a ‘weapon’ and if a release of kinetic energy is required. In doing so, it discusses subsequent agreements on the meaning of a ‘use of force’ in article 2(4), including the 1970 Friendly Relations Declaration, the General Assembly’s 1974 Definition of Aggression, 1987 Resolution 42/22 and the 2005 World Summit Outcome Document. It also examines in detail the travaux préparatoires of the Friendly Relations Declaration regarding the definition of ‘force’ in article 2(4) and arguments for and against a broad interpretation. This chapter concludes that ‘use of force’ article 2(4) refers only to physical force and not to non-physical forms of coercion, that it is not necessary that a ‘weapon’ be used nor is it required that kinetic energy be released, and that physical means are not essential for an act to constitute a ‘use of force’, as what counts are its physical effects.

Chapter 2’s purpose is to provide some focused discussion on the meaning of the ‘force’ prohibited in Article 2(4) and customary international law. In seeking to expose the uncertainties regarding this particular term, various factors or common elements of ‘force’ are distilled. After looking at the prohibition of force in the context of the principle of non-intervention, the chapter moves on to look at the type of force that the prohibition is concerned with and, concluding that it is ‘armed force’, then moves on to attempt to distil the key elements of such force, including whether it is the means used or the effects created which is of importance, and whether force can be used indirectly. The chapter then addresses the ‘gravity’ or severity aspects of a use of force, in particular by distinguishing it from an armed attack or act of aggression, but also by examining whether there is a level of force – or de minimis threshold – below which an action falls out of the remit of the prohibition. Finally, and having distilled the key practical components of a prohibited use of force, the chapter focuses upon the mens rea component.