A crime against peace was defined as "the planning, preparation, initiation or waging of a war of aggression, or a war in violation of international treaties, agreements or assurances, or participation in a common plan or conspiracy for the accomplishment of any of the foregoing..."

Unfortunately, no definition of "war of aggression" has ever been adopted (of especial relevance to our conversation, the UN and the ICC are still fighting over definitions to use). In lieu of a definition of "war of aggression" most states have conducted themselves in accordance with the second definition of crime against peace, "a war in violation of international treaties, agreements or assurances."

Neither the Afghanistan War nor the Iraq War disturbs this second definition. Given that, it's going to be very difficult to convict anyone in the Bush Administration for a crime against peace.

Regarding waterboarding during World War II, that case is distinguishable for three reasons. First, Asano (the fellow in question) was convicted of beating and clubbing prisoners, "water torture", burning with cigarettes, and (oddly enough) strapping prisoners head down on a stretcher. Second, Asano's victims were Prisoners of War. Third, Asano stole from supplies intended for the POWs (this crime was itself a crime against the laws of war).