Preemptive strike without formal declaration of war

https://en.wikipedia.org/wiki/Authorization_for_Use_of_Military_Force_Against_Iraq_Resolution_of_2002

U.N. approval

Getting UN approval is a "nice to have", but it doesn't guarantee anything. A war of aggression would probably be something that could be prosecuted in the ICC as a crime of aggression, but to prove it's a crime of aggression you need to prove that there was no "just cause for self-defense". The whole basis of the US justification for attacking Iraq was that Iraq was involved in terrorism against the USA. So, to prove that it wasn't a war of self-defense, you'd not only have to prove that Iraq had no connection with any kind of terrorism against the USA and had no intention of it in the future, but that the US leadership knew that that was the case and invaded under false pretenses.

At this point we know that Iraq didn't have WMD, but can you really prove that the US leadership wasn't so deluded that they thought that Iraq genuinely didn't have WMD? The whole aftermath of the invasion involved a lot of embarrassing searching for WMDs that the US was sure were there. The US was constantly announcing that they were closing in on the WMDs, but every site they searched turned out to be nothing. If they'd known there really weren't any, they probably would have just gone ahead and planted some evidence. Instead, they kept looking and looking and claiming they were sure it was there somewhere.

Besides, the US has a veto on the UN security council, so they couldn't recommend prosecution or anything because the US would just veto the resolution.

were charged and found guilty of war crimes in foreign courts

Which foreign courts? Which war crimes in particular?

ICC (international criminal court) decided not to pursue the matter even though they had ample evidence cause Murica.

"cause Murica" is your reading of it. They had the option to charge Bush, but they didn't. One reason for that might have been that they knew they'd never be able to get their hands on the US officials they could have charged, and that the US might react really badly to the charges. But, another reason might be that they knew they'd never be able to get a conviction, because the bar to convicting officials is very high at the ICC.

"Evil shit" isn't the same as "crimes". The Bush admin did plenty of evil shit, but it's very hard to prove they broke any specific laws. Instead, because they managed to scare the shit out of the US, congress and the senate kept giving them as much authorization to do whatever they wanted. As for international laws, those are very rarely used, especially against superpowers, and the bar to proving anything is very high.