Its really pretty simple. Because there is so much oil under Iraq we can not let its control be up to chance, or fall into the the hands of Iran - most likely not by an outright attack, but by a war of religious factions supported by Iran. Oil fuels the tanks and the planes, as well as most economies in this world. Iran can't have it - trust me. Just watch the Dow index when a pipeline explodes.
Iraq needs to have a strong enough military to defend a government that is alien in the region (i.e. democracy). It can't be a token army, especially with Iran having nuclear capabilities. Such an army, capable of dending Iraq, would be a threat to Isreal. So before they can have it, we have to make damn sure they're on our side. They have to like western society. Thats going to take awhile.
The realistic option is not to expect them to be able to defend themselves, in which case we'll be permanent guards of Iraq.
That is the likely scenario, and it aint gonna be cheap.