The war in Iraq brought the lingering difference between Americans and Europeans into stark relief. Europeans were against the war, for the most part and Americans were for it, for the most part. It is, ironically, a reversal of world views. During the 18th and 19th centuries, the Americans were the ones espousing the importance of international law and the need for subtle diplomacy while the Europeans who made use of raw power on the international stage.
In the late 20th century, with the Soviet Union no longer a threat, the weakness of Europe forced it to take the old American strategy while the unchallenged might of the United States made it take on a different view.
This culminated with Iraq. As Robert Kagan put it, a man armed only with a knife may come to a different decision on what to do about the bear than the man armed with a rifle. From Europe's point of view, Iraq, led by Saddam Hussein, was a threat but not an intolerable one. Removing Saddam was beyond the ability of the European military powers without great sacrifice.
By contrast, the Americans made a different calculation. Saddam could be likely removed with only a couple thousand American casualties. So why should this lingering threat be allowed to continue, especially after 9/11? And events bore this out. The United States was able to march into Baghad and remove Saddam with only a few hundred combat deaths. The 2 years since had brought on several hundred more deaths due to the "insurgency". A number that is pretty unimpressive when one considers that America's drunk drivers are more effective killers than the armed guerilla's that make up Iraq's insurgency.
This essay isn't to argue that the Americans were right to go into Iraq. Only that from the American perspective, if a significant thorn in the side and lingering threat to its security can be removed so easily, then why not do it? The Europeans, by contrast, really didn't have such an option. Realistically, they had to put up with Saddam no matter what. It is far easier to rationalize his existence than to simply admit that there is nothing they could do about it.