This chapter assesses the powers of the Security Council in three stages. First, it introduces the scope of the Council’s powers. They are potentially far-reaching, although within a particular field – the maintenance of international peace and security. The chapter then examines specifically the Council’s practice and discretion with respect to determining the existence of a threat to the peace, breaches of the peace, or an act of aggression, under Article 39. Finally, it addresses whether such determinations are subject to judicial review.

Section C examines the law on the resort to the use of force, specifically nuclear force. Accordingly, it contains Rules and Commentaries on the prohibition of the use or threat of nuclear force; the right of individual or collective self-defence, including its limitations; and enforcement measures decided upon by the UN Security Council, including those executed by regional organisations.

Chapter 3 considers the meaning of an ‘act of aggression’, defined in Article 8bis (2) of the Rome Statute. It opens with a comparative analysis of the terminology employed in the UN Charter to describe inter-State armed force. It then summarises the shortcomings of the Definition of Aggression annexed to General Assembly (GA) Resolution 3314 (1974), which provided a basis for Article 8bis (2). Among other things, it is demonstrated that the 3314 Definition does not reflect the understanding of an act of aggression seen in the practice of the UN Security Council, GA, or the International Court of Justice. It is also explained how some of the 3314 Definition’s shortcomings were exacerbated by the selective use of its provisions in Article 8bis (2). In addition, the chapter considers the treatment of the acts of non-State actors, and cyber operations and emerging weapons technologies. Finally, the chapter analyses the status of the ‘Understandings’ adopted by the ICC Assembly of States Parties alongside the aggression amendments, and their effect on our understanding of the definition of the State act element of the crime.