The Nuremberg trials dealt with crimes of a scope that humanity had not seen before, at least not in the modern world. To try the Nazis for violating treaties or comitting simple murder did not address the true horror of what they did.
Also if you can't understand the conept of ex post facto law how can you continue this conversation?
I understand the concept quite clearly, but concepts of law meant to apply to crime within the framework of a functioning country do not really work in a situation like post-war Germany.
The occupying powers were faced with a situation where they could (i) exact victor's justice by executing everyone in the German government above a certain rank (ii) do nothing to punish the unprecedented crimes before them or (iii) try to come up with some sort of process that would punish the guilty while clearing non-criminal members of the German government.
The end result wasn't perfect, but it did a pretty good job of punishing those members of the German government responsible for mass murder, aggression against other nations and genicide, among other crimes.
Judicial relevance, eh? slippery slope time...
Actually you are mistaken. Genocide of Russian Christians took place EARLIER and was on MUCH BIGGER scale. Bolshevik murderers were among the judges in Nuremberg and this fact alone made the trial a mockery of justice.