Although a similar crime of aggression was agreed upon at the Rome Conference, a definition of the crime and how it should be investigated did not achieve consensus and was only included in the statute on the last day of the 1998 conference. (slaw.ca)
So the question was whether the customary international law crime of aggression was a crime in English law, although not contained in an English statute. (ejiltalk.org)
The Court came into effect in 2002 with a mandate to try individuals for four big crimes: genocide; crimes against humanity; war crimes, and the crime of aggression. (blog.politics.ox.ac.uk)