Because the U.S. chose to get involved in the affairs of Iraq, we now have a moral obligation to set up an actual free society there. If we replace one tyranny with another, we have betrayed our own highest principles. Ways in which we could fail include, but are by no means limited to, the following:

If we install or allow the formation of some kind of a government where "tribal" membership is very important and tribal leaders have a lot of influence, you can bet that we will just be paving the way for the warlords and their private/family/tribal armies to carve up the place in a few years.

If we install or allow the formation of any kind of theocracy, we will produce a government that will eventually be much more likely to attack the U.S. than the one we just removed.

If we set up a government that has no actual support from its people, then it will either require our continuous military help just to stay in power, or resort to totalarian measures of its own, or fall apart and be replaced by something much worse. Or all three, in that order.

If we decide to keep Iraq under our thumb and run their affairs for an indefinate time, that would be naked colonialism, a repudiation of the principles of the American revolution itself (unfortunately, far from the first in our history).
me