The Blogosphere is all abuzz with that question these days, especially now that Bush has admitted what we've all known and suspected for some time: We will be there for years and decades to come, on permanent bases.
I think you have to go back to what Paul Wolfowitz said in his famous
Vanity Fair interview:
There are a lot of things that are different now, and one that has gone by almost unnoticed--but it's huge--is that by complete mutual agreement between the U.S. and the Saudi government we can now remove almost all of our forces from Saudi Arabia. Their presence there over the last 12 years has been a source of enormous difficulty for a friendly government. It's been a huge recruiting device for al Qaeda. In fact if you look at bin Laden, one of his principle grievances was the presence of so-called crusader forces on the holy land, Mecca and Medina. I think just lifting that burden from the Saudis is itself going to open the door to other positive things.
This is about the Saudis in a big way. Our troops there were destabilizing the Saudi regime by inciting Al Qaeda. So what to do with our massive presence in the region? You can't put them in another friendly country it will cause them too much grief. And the less-friendly won't have us. So you either leave or knock somebody off. Saddam was the obvious choice. That move was supposed to be easy and popular.
And of course there's the money: Just consider how much of the the federal treasury has been directed to companies like Haliburton and you see what a success the Iraq war has been for its benefactors...