Unfortunately, for them, their lack of imagination has led them to create missions that are either unnecessary (rid Iraq of WMD) or silly (free the Iraqi people by oppressing them with a huge occupation army). Often both. As each mission statement is proven to be a joke the Iraq mission is morphed into something different.
Being there to protect, obviously, our own interests, which are to go after al-Qaida and make sure that al-Qaida can never use Iraq as a safe haven from which to plan attacks against us ~ national security adviser, Stephen J. HadleyThe current mission statement fall into the silly category. Al Qaeda is not a significant presence in Iraq with only about 1000 adherents in the country. Their pathetic size does not justify a major occupation army. If they are the target then a 10,000 man mobile attack force would be more than sufficient to defeat them. Al Qaeda is simply the latest excuse, the latest fiction.
The goal now is what is has always been - maintain a colonial force in the heart of the Middle East to swing the regional balance of power towards our allies, the Saudi princes. The mission is to keep a lid on an Iraqi Civil War (lest the Iranian side prevail) while installing a puppet pro-American Shi'ite government.