If wrecking the towers was an act of war, then it wasn't criminal

Not so. If it was an act of war it was a criminal act of war, and should be treated as such. As should all war crimes, whichever side carries them out.

And yet I understand that there is still opposition by United States to there being set up a standing Internationbal War Crimes Tribunal.

People who are responsible for war crimes are the enemy, whoever they are and whatever flag they may have brought shame on.