I'm very much a layman, and an Englishman rather than American, with an interest in political philosophy. I was reading through the US constitution the other day, and one sentence in Article I, Section 9 stood out to me:
No Bill of Attainder or ex post facto Law shall be passed.
With respect to ex post facto laws in particular, this made me wonder about the legality of the Nuremberg Trials. One of the key points made by the defendants at Nuremberg was that their transgressions - the mass killings, the conspiracies to wage wars of conquest, etc - were not crimes under German law at the time they were committed.
The Nuremberg trials represented an uncomfortable fusion of American, British and Soviet law. To the best of my knowledge, there is no straightforward ban on ex post facto legislation within British law, although there is a vague consensus that such laws are unjust and are to be avoided. Whatever the articles contained within it, the Soviets in this period only seemed to treat their constitution as a set of guidelines and aspirations. But it strikes me as odd that the Americans would be willing to overlook such a clear ban, since they, more than any other nation, have tended to view their constitution as a kind of sacred text.
I anticipate that one response to my question will be that the US constitution applies to domestic matters, and not to international law. This seems correct at first glance, until we read in Article I, Section 8 that one of the powers of Congress granted by the constitution is 'To define and punish Piracies and Felonies committed on the high Seas, and Offences against the Law of Nations'. But I'm sure that there are many layers of subtlety that I'm failing to grasp here, and I'd be grateful to anyone who could put me straight.
I think I should close with the disclaimer that, in calling into question the legality of the Nuremberg trials, I don't intend to exonerate any of those who were convicted. They were certainly criminals in the moral sense of the word, and, putting questions of legality to one side, the world was better off without them in it.
Regarding the matter of whether this question is a duplicate, I'll quote one of the comments below:
The linked question is relevant, but not a duplicate. It briefly mentions the question of whether the trials operated under ex post facto laws, but devotes more space to other purported issues. None of the current answers address this issue. This should not be closed as a duplicate.