This chapter argues that, although the Nuremberg proceedings, otherwise known as the International Military Tribunal (IMT), were heavily focused on the crime of aggression, international criminal law’s (ICL) emphasis has shifted dramatically. Since the reinvention of ICL in 1990s, it has foregrounded atrocity. ICL is often presented as following a smooth trajectory, but actually there was a reversal or massive shift, from a priority on aggression to its near exclusion. The focus on atrocity—and the aspiration to make war ‘clean’—may humanize war rather than stigmatize it, and perhaps even enable war instead of limit it. This chapter suggests, as an explanatory hypothesis, that Nuremberg took place during a ‘passing window of plausibility’: the USA has generally opposed the criminal prohibition of aggression, either because such a system might demand US intervention or because it might pass judgment on US interventions. Circumstances aligned to allow the Nuremberg proceedings, after which ICL stalled again, and the switch to an atrocity focus helped fill the resulting void.