Chapter 4 considers what is required for an ‘act of aggression’ to amount to the State act element of the crime of aggression – namely that it constitutes, by its character, gravity and scale, a manifest violation of the UN Charter. The chapter analyses the manifest violation threshold in order to provide meaning to it. A number of criticisms associated with the definition are rebuffed, particularly that the definition violates the nullum crimen sine lege principle and that the definition has given a green light to ‘lesser violations’ of the UN Charter. At the same time, on the basis of an exhaustive survey of State practice and opinio juris, it is demonstrated that the definition of the State act element of the crime of aggression under the Rome Statute is inconsistent with the crime’s customary definition, and that it will require the judges of the ICC to resolve controversies associated with unclear rules surrounding the prohibition of the use of force. The chapter finally considers the grounds for excluding criminal responsibility under Article 31 of the Rome Statute in relation to the crime.