"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." International Military Tribunal, Nuremburg, The Trial of German Major War Criminals, Judgment: 30th September, 1946 - 1st October, 1946, at "Conspiracy and Aggressive War," p.13.
To access the full quote within the official Nuremburg Tribunal records go to the third and fourth paragraphs under the heading "THE COMMON PLAN OR CONSPIRACY AND AGGRESSIVE WAR."
For more about the illegality of the Iraq War, see the Western States Legal Foundation page on this subject.
Even Richard Perle admits the war was illegal; see "War critics astonished as US hawk admits invasion was illegal."
"She [America] goes not abroad, in search of monsters to destroy. She is the well-wisher to the freedom and independence of all. She is the champion and vindicator only of her own. She will commend the general cause by the countenance of her voice, and the benignant sympathy of her example. She well knows that by once enlisting under other banners than her own, were they even the banners of foreign independence, she would involve herself beyond the power of extrication, in all the wars of interest and intrigue, of individual avarice, envy, and ambition, which assume the colors and usurp the standard of freedom. The fundamental maxims of her policy would insensibly change from liberty to force....She might become the dictatress of the world. She would be no longer the ruler of her own spirit."
John Quincy Adams, 1821