George W. Bush's ongoing peacekeeping and democracy building mission in Iraq annoys a lot of people for a lot of reasons, but it does make some sense in the context of Wilsonian diplomacy where you accept the concept that it is the job of the great powers of the world to maintain peace and order so that the poorer and more chaotic nations of the world can become civilized and develop more advanced and productive economies.
This idea, which is behind the efforts of many of the efforts of international groups like the UN and the Council on Foreign Relations, assumes that the world will be a better place for everyone if the poor folk of the world are not starving or fleeing from war and violence or spreading trouble by conquering their neighbors. It makes sense if you accept that trouble anywhere in the world can spill over or have fallout and negative consequences for the wealthier and more powerful nations no matter where they are located.
It makes much less sense if you assume that there are more specific limits to national self interest or limits to the amount of money, men and resources you can afford to spend on a missionary foreign policy. As a general rule a liberal foreign policy should at least be somewhat guided by pragmatic self interest because it makes no sense to ruin and bankrupt your own country for the benefit of others.
Spreading peace and promoting economic development makes some sense, so long as the cost is reasonable and the benefits are concrete and likely to be reaped in the foreseeable future. Stable nations with relatively modern and productive economies make good trading partners. If you accept the idea that one of the roles of government is to promote the general welfare (as stated in the Constitution) through maintaining a healthy environment for international trade, then giving a hand up to potential partner nations seems reasonable.
It is also true that if you allow oppression and violence free reign, then the more primitive and troubled nations of the world will spread chaos and poverty in the form of disease and refugees and even invasion and conquest. They can even export violent and destructive ideologies. Improving conditions in these hotspots makes life better for everyone.
All of this informs the decision to become embroiled in Iraq, a country with great economic potential in the heart of the most troubled and unstable region in the world. Stabilize Iraq and you help stabilize the region, allowing economic development and reducing the threat posed to the rest of the world.
The problem with this approach to foreign policy is that the threat posed and the benefits to be reaped in the middle east do not necessarily benefit the United States and certainly not to a degree to justify a cost of a trillion dollars.