1) The European powers created artificial borders in the colonial world which do not accurately reflect national boundaries.
2) War is probably justified against any the leadership of nations who use force to disrupt international peace. For examples of this, we could look either to the substantial unanimity around the world supporting the first Gulf War, or to Chapter VII of the U.N. Charter itself, supporting the right of nations to aid the self defense of victims of war.
3) The War in Iraq is not justified (two years after they all voted for it) since there were no weapons of mass destruction, no terrorists, and no breach of the international order.
Focusing on the third tenet, I would like to ignore for now the first claim (that the absence of WMD removes the justification for war), since it requires separate analysis. After all, War probably can be justified by a legitimate RISK of WMD, even if that risk (like any odds calculation) proves to have been likely but false. I would also like to ignore the second (the absence of terrorists in Iraq) since it has proven so obviously absurd as to need no discussion here (even CNN can find the terrorists there, on a daily basis).
I would like to briefly touch on the liberals' hypocritical conception of international law's understanding of the prerogative of the United States to assist in collective self defense. Their standpoint is clearly laid out by the change in Democratic/European opinion over the two Gulf Wars. The first Gulf War, aside from everything else, was justified to them because Saddam Hussein initiated the era of War. The default is to be a ban on war, but in invading Kuwait, Saddam fractured the peace so harshly that he could be dealt with forcefully. This justification for War is clearly preserved in the U.N. Charter, where even the absence of a Security Council resolution can lead to war where nations assist other peoples in Self Defense.
Fast forward to 2003. Here, Saddam Hussein had not invaded another country, so the liberals distinguished between Gulf War I and Gulf War II. But let us consider what the nature of War is. War is an attempt by a government to use force to extend its laws past the boundaries where they are entitled to exist. It is (to keep with the theme around here) an act of ultimate lawlessness, using violence to throw out the most fundamental legal characteristics of another system (who is in charge and who decides who gets to be in charge). While Saddam Hussein certainly enforced his will against many peoples who, behind the veil of ignorance, might be considered separate nations from Iraqi Sunnis, the happenstance of the Iraqi border deprived this brutalism of the label "War."
Here's the point. If the borders in areas formerly under European control are not sensible, than the insistence on requiring acts of War to cross these artificial boundaries is also not sensible. Saddam Hussein did worse thinks to the Shiites and Kurds in Iraq then he ever dreamed of doing to the Kuwaitis. He tortured, maimed, and robbed them. He assassinated the leadership of these peoples, and did the best he could to wipe whole populations of them off the face of the Earth. In a world with sensibly drawn maps, these acts probably would have met the liberal definition of War, removing all debate as to the propriety of U.S. liberation actions.
Those of us who are concerned with Law have less license to pay attention to artificial boundaries. Saddam Hussein has been making war against the Kurds and the Shia for the extent of his reign of terror. He used force and murder to substitute their ability to have rights and laws for his own desires. Formalistic attention to the Europe-drawn border system in deciding the rights of abused peoples is neither a normal nor a consistent approach.