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.

The Vienna Convention on the Law of Treaties remains the yardstick for the interpretation of treaties. International tribunals generally base their decisions on Convention rules, although, as different areas of law with their own dispute settlement systems have developed, there are also particular approaches. The Convention rules are contained in Articles 31--33, and the chapter focuses on these. Each element of the general rule of interpretation in Article 31 and the supplementary means of interpretation in Article 32 is analysed, including the role of subsequent agreement or subsequent practice. Further sections summarise other supplementary means of interpretation and look at treaties in more than one language.

Chapter 6 undertakes a detailed study of the jurisdictional provisions that govern the crime of aggression (Articles 15bis and 15ter). In contrast to the ICC’s ordinary jurisdictional regime, the provisions allow States Parties to shield their nationals, and entirely insulate the nationals of non-States Parties, from prosecution, except in the case of Security Council referral. The chapter considers the legality of this regime. It then examines the different interpretations of the jurisdictional provisions offered by States and commentators, and the effect of the resolution under which the ICC Assembly of States Parties (ASP) activated the Court’s jurisdiction in 2017. Based on a detailed examination of the jurisdictional provisions, relevant ICC ASP resolutions, and the governing amendment provision, the chapter sets out exactly who is subject to the ICC’s jurisdiction. It also examines the requirements of opt-out declarations, the role of Security Council determinations as to the existence of an act of aggression, the expected operation of the Pre-Trial Division filter, and the effect of a Security Council referral.