Starting an illegal war of aggression is a crime for which some people were convicted in Nuremberg Tribunals.
However, all the soldiers fighting in such an illegal war of aggression are not considered responsible for that crime. For "simply" fighting a war and killing uniformed enemy soldiers "I was just following orders" actually is a legitimate excuse, and should not result in any convictions (of e.g. murder) unless they commit some explicit war crimes, e.g. murdering captured civilians.
> For "simply" fighting a war and killing uniformed enemy soldiers
But, that is not how those wars happen. Like, literally none of those wars was limited to killing uniformed enemy soldiers. That might be Hollywood idea of war.
But pretty much all of real ones involved units killing/torturing locals. Sometimes more and as a strategy, other times less so. But it happens literally every time. It is not that all armies are the same, they are not. Some are much much better then others.
But, in general, worldwide, "simply" fighting a war and killing uniformed enemy soldiers is not what war is.
While none of the wars was limited to killing uniformed enemy soldiers, for the vast majority of soldiers fighting those wars (even if illegal wars of aggression) their personal actions were limited to killing uniformed soldiers and they are not responsible or liable for any war crimes, as they did not perform them.
However, all the soldiers fighting in such an illegal war of aggression are not considered responsible for that crime. For "simply" fighting a war and killing uniformed enemy soldiers "I was just following orders" actually is a legitimate excuse, and should not result in any convictions (of e.g. murder) unless they commit some explicit war crimes, e.g. murdering captured civilians.