A hybrid legal discipline dealing with the relationships between the right to punish and state sovereignty, international criminal law (ICL) overturns classical conceptions of the state, of law and of justice. Its existence, its foundations, its scope and its effectiveness are all determined by the outcome of an attempt – which has proved more or less successful throughout the different phases of its evolution – to reconcile it with the two founding principles of the modern state, sovereignty and legality, inherited from the philosophy of the Enlightenment. The path leading to the concrete realisation of ICL was a tortuous one. Adopting a historical perspective helps us to retrace two key stages in its development, on either side of the pivotal moment represented by the creation of the Ligue of Nations (LoN): the starting point marked by the 1919 Paris Peace Conference and the immediate aftermath of World War I; and the turning point marked by the work of the LoN and international legal doctrine in the interwar era. These two crucial phases saw a string of abortive initiatives which, rather than failures, can be interpreted as a series of necessary transformations for the emergence, in the fullness of time, of a new discipline in its own right and, more generally, a profound change in the global legal and judicial order.