I'd like to point out that many of the atrocities committed by the Nazis in the early 1940s were not, under their own laws, considered criminal. Nevertheless, it was possible to punish them (rather, a select handful who were egregious enough and also not directly useful enough to the prosecuting nations to recruit, but that's another point) for crimes against humanity.

Broadly speaking, the legal framework is there. Its the legal willpower that is lacking.