Milosevic, Saddam, and their ilk in Rwanda were the last of the twentieth century's mass murderers. As long as the civilized world has the power to stop such men it is a moral disgrace to refuse to do so. This is why "Bush's war" in Iraq was the right thing to do even were there no reason to believe that Saddam had WMD. It was as right for the U.S. to unseat Saddam as it was to stop Milosevic, as it was right for the U.S. to enter the war in Europe against the Nazis.
To stand by and let mass murder take place when one has the power to stop it is to be either complicit in the crime of genocide or to be craven in the face of it. In either case, it is a shameful stain on a nation's honor.
Yet much of the left is outraged that the United States employs military might against tyrants around the world when other measures fail. It's not that they object to force itself, mind, because throughout the twentieth century they never uttered a peep when the Soviets used it to keep their satellites in line. Rather they recoil from American use of force because they loathe this country and its values and do not wish to see either its power or its honor increase vis a vis the rest of the world.
A strong, dynamic and just U.S. is a reproach on all they believe and all they have given their life to convince others of. It's a reproach that they cannot bear, and it's one reason why they are not dismayed by American setbacks or when Americans betray their own principles, as at Abu Ghraib. Rather many on the left revel in these meager confirmations of their basest suspicions. It's really quite pathetic.