If the U.S. is obligated to liberate the oppressed by force, there is surely no reason to stop with Iraq. Assuming that oppressed peoples are equal wherever they may be, then we should surely seek to overthrow the governments of Saudi Arabia (whose human rights record is every bit as bad as Saddam's), Turkey (whose treatment of the Kurds has been considerably worse than Saddam's), Columbia (whose scorched earth tactics and death squads are perhaps unparalleled right now anywhere on Earth), and Israel, whose treatment of Palestinians continues to represent a form of not-so-subtle ethnic cleansing.
But of course we won't invade any of those places to free the persons oppressed by the respective governments, and the reason is obvious: because we are allied with those nations, and implicated in the oppression of the peoples mentioned. They can torture, imprison, rape, behead in public, crush with tanks, and otherwise slaughter as many of these as they choose and nothing will happen to them. Because we are not about liberation. It is simply an excuse we use to help us sleep better at night, and because we think the people of the world are so stupid as to actually believe it....
And let us not forget that even with the support of the Iraqi people, U.S. actions have planted the seeds of further terrorism, as millions throughout the Middle East seek revenge for what they perceive as an American power grab and anti-Arab, anti-Muslim crusade.
After all, we thought all was well after our liberation of Kuwait in 1991, too. Kuwaitis were certainly happy, I suppose. But a certain Saudi national was not. He saw the stationing of U.S troops in his nation as an affront to his religion; an unholy incursion onto Muslim land. He also saw the human costs of the war that liberated Kuwait as an unacceptable massacre, and the sanctions that came after it as de facto genocide. And at least nineteen others agreed with him.
Such is the inertia created by this kind of liberation.