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.

For various reasons, Glaser must be understood as a significant scholar of international criminal law. Already in the 1920s, he wrote about the idea of international criminal justice and the creation of an international criminal court. He devoted his main research after World War II to this emerging discipline. He was the first academic who collected various forms of sources and opinio juris to give scholarly support for the development of a new discipline of ICL to be studied at institutions of higher education. At the same time, Glaser was a lawyer involved in some of the political and judicial turmoil of his times especially in Poland. He supported and represented the repressed members of Poland’s political opposition before Polish courts before World War II, he also fought against impunity for grave crimes through his work in the UNWCC during the war, and he provided advice on the draft for the Convention on the Non-Applicability of Statutory Limitations for Grave Crimes.