This chapter begins by discussing how the crime of aggression differs from all other core international crimes in being inextricably linked to an act of aggression by a state against another state. It then turns to a discussion of the history of the crime of aggression, including its inclusion in the Statute of the International Criminal Court (ICC). It covers the definition of the crime of aggression as set out in Article 8bis of the ICC Statute, as well as its relationship with other crimes. It also examines the material elements: (1) by a perpetrator in a leadership position in a state (2) who has participated (3) in an act of aggression by the state (4) which ‘by its character, gravity and scale, constitutes a manifest violation of the Charter of the United Nations’. It also explains the mental elements as set out in Elements 4 and 6 of the ICC’s Elements of Crime; the jurisdiction of the ICC over aggression, including the role of the Security Council; and the implications of an ICC prosecution of the crime of aggression.

This chapter challenges the dominant paradigm of a ‘use of force’ in article 2(4) of the UN Charter as a coherent concept by giving examples from the subsequent agreement and subsequent practice of States showing that none of the elements of a ‘use of force’ is strictly necessary for the definition to be met. It examines anomalous examples which lack the elements of physical means, physical effects, gravity or hostile intent, taking as a basis of analysis certain acts listed in the 1974 Definition of Aggression: military occupation (article 3(a)), blockade (article 3(c)), mere presence in violation of a Status of Forces Agreement (article 3(e)) and indirect use of force through inter-State assistance (article 3(f)) or through non-State armed groups (article 3(g)). It also examines anomalous examples of non-‘use of force’: acts which appear to meet the criteria but are not characterised as such by States in their subsequent practice. These include forcible response to aerial incursion and purported maritime law enforcement. This chapter then offers possible explanations for these anomalous examples and discusses the implications for the interpretation of a prohibited ‘use of force’.

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.