But in principle, since 1945, no one can evade personal responsibility for a clearly unlawful act by pointing to superior orders.
Really though, this is a stupid comparison. People generally remember the Nuremberg trials in the context of the Holocaust -- genocide, slave labor, and all the horror that accompanied it. Colin Powell did nothing remotely close to any of that, and while there is plenty of criticism to throw at the US for the invasion of Iraq there was no genocide, no ethnic cleansing, no slave labor camps, and a dozen US soldiers were thrown in prison by the US for having tortured and humiliated prisoners under their command.
> Operating partly under the dictates of the old prize rules, the U-boat commander, Korvettenkapitän Werner Hartenstein, immediately commenced rescue operations. U-156 broadcast her position on open radio channels to all Allied powers nearby, and were joined by the crews of several other U-boats in the vicinity.
>[...] En route, the U-boat was spotted by a B-24 Liberator bomber of the US Army Air Forces. The aircrew, having reported the U-boat's location, intentions and the presence of survivors, were then ordered to attack the sub.
>[...] The commanders of the Kriegsmarine were quickly issued the Laconia Order by Grand Admiral Karl Dönitz, which specifically forbade any such attempt and ushered in unrestricted submarine warfare for the remainder of the war.
>[...] During the later Nuremberg trials, a prosecutor attempted to cite the Laconia Order as proof of war crimes by Dönitz and his submariners. The ploy backfired and caused much embarrassment to the United States after the incident's full report had emerged.
https://en.wikipedia.org/wiki/Laconia_incident
It's also specifically not about genocide vs non-genocide but about legal vs. illegal orders. Just because you represent the US of A you cannot lie to the whole world to start a war. I mean, you can as evidenced by Powell, but a lot of people will spit on your grave for that.
Suppose he fled the US for some neutral country, resigned his position, and made a public announcement that he knew that the case for war was fraudulent and that the international community had an obligation to stop the US. What would have happened?
Suppose he simply resigned to spend more time with his family the night before his speech to the UN.
Suppose he did not decide (as he did in real life) that he was going to support the president in waging war without the support of the UN, and he told the world as much.
Suppose he insisted on having more time to review the material himself before making the speech to the UN.
There were a number of times where he could have changed the course of history. Yes, it would have come at significant personal cost to himself and perhaps significant embarrassment of America on the world stage. If you're unwilling to do that for a raise, sure, that seems reasonable; if you're unwilling to do that to prevent tens of thousands of deaths, you are absolutely morally culpable for those deaths.
The invasion of Iraq on false premises was itself a war crime, and the killings of Iraqi soldiers was just as immoral as killing civilians. Quoting one of the rulings at Nuremberg: "War is essentially an evil thing. Its consequences are not confined to the belligerent states alone, but affect the whole world. To initiate a war of aggression, therefore, is not only an international crime; it is the supreme international crime differing only from other war crimes in that it contains within itself the accumulated evil of the whole."
Powell had a hard choice, yet life often puts us in such situations. He should have resigned, in my view. If he did not, he shares in the responsibility for what was done.
Someone else could have been the messenger, and Powell could have also resigned in protest and become a whistleblower.
Instead he lied, deceived the public, and unequivocally supported the wars.