Chapter 2 counters each of the policy arguments that have been mounted against the criminalisation of State acts of aggression, including arguments relating to uncertainties surrounding the prohibition of the use of force; that acts of aggression are too complex an episode for adjudication by a criminal tribunal; that the criminalisation of aggression will politicise the ICC; that there is a risk of a fragmentation of relevant rules of international law caused by differing decisions of the ICC, the UN Security Council, and the International Court of Justice; and claims that the criminalisation of acts of aggression will have a chilling effect on States’ willingness to use force when they should (namely, to prevent atrocity crimes). The dissection of such arguments demonstrates that each objection is misguided and sets out the case in favour of criminalisation. The chapter also assesses the geopolitical significance of the crime, considering the extent to which the criminalisation of the unlawful use of force represents a shift in the balance between power and law. Finally, the chapter examines the likely impact of the crime of aggression on the rules of the jus ad bellum.