Were the Nuremberg trials legal with respect to the US constitution?

Question

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.

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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.

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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.

Answer 1

The US Constitution has no bearing on the Nuremberg Tribunals

The tribunals were not conducted under US law, they were conducted under an international treaty going by the snappy title of No. 251 Agreement by the Government of the United Kindom of Great Britain and Northern Ireland, the Government of the United States of America, the Provisional Government of the French Republic and the Government of the Union of Societ Socialist Republics for the Prosecution and Punishment of the Major War Criminals of the European Axis. This was signed for the United States on 8 August 1945 by Robert H. Jackson who later served as chief prosecutor on the tribunal.

The laws of the United States allow three mechanisms for the US to join international treaties: treaties, congressional-executive agreements and executive agreements. I do not know what mechanism was used but AFAIK, no contemporary challenge was made that the US lacked the power to or hadn't properly acceded to the treaty. This is the extent to which US domestic law would be applicable.

The US recognises that there are other laws and other courts than merely US ones and that their citizens may be subject to those laws and serve on those courts. Indeed, there are circumstances where US courts will apply foreign law to a case; when that happens US law applies to the procedure but the substantive law of the other jurisdiction is applied to the facts.

There is no doubt that the legal basis of the Nuremberg Tribunal was suspect and controversial and this was recognised at the time as this April 1946 The Atlantic article demonstrates. However, there was never any doubt that US law (or UK, or USSR, or French law) was totally irrelevant. I suggest you ask a new question focusing on the role of "Bill of Attainder or ex post facto Law" under international law.

Answer 2

It would come down to the question if the US Constitution binds the US government in how they enter and apply instruments like the London Charter aka Nuremberg Charter or for that matter the unconditional surrender agreement with Germany. Neither of them is a law or bill, exactly.

There have been plenty of cases where the US found it legal (constitutional?) to apply different principles abroad than they do at home, recently regarding the Guantanamo detainees.