The International Criminal Court (ICC) [official website] on Tuesday assumed jurisdiction over the crime of aggression, coinciding with the twentieth anniversary of the ICC’s founding under the Rome Statute [text, PDF].

The crime of aggression, under which politicians and military leaders can be held individually responsible for invasions and other major attacks, comes into force at the international criminal court, reviving global legal powers last exercised at the Nuremburg and Tokyo war crimes trials of the 1940s.

The wording defines aggression as “the planning, preparation, initiation or execution, by a person in a position effectively to exercise control over or to direct the political or military action of a State, of an act of aggression which, by its character, gravity and scale, constitutes a manifest violation of the Charter of the United Nations.”

Supporters hope that this will offer some prospect of deterrence and some degree of certainty in identifying criminals. However, the new offense cannot be enforced retroactively.

The ICC will only have direct power of prosecution over the states that have ratified the recently defined crime. Only 35 countries have done so, making their leaders liable to prosecution if they start a war. The UK is not among them, and the US is not a member of the ICC.