Why US soldiers are still dying in Iraq
If the US were to withdraw, Saudi Arabia would be forced to fill some of the vacuum for their own interests in offsetting Shia power in Iraq and the region. The most likely way the Saudis would accomplish this is by arming Sunni groups in the Iraq civil war. (No surprises yet.)
But then Clark pointed out that the Saudis would likely fund the best trained, most committed Sunni fighters, Al Qaeda.
So, the argument is that US soldiers are dying because "our ally" is threatening to fund, arm, and train the next generation of Al Qaeda who, after fighting US marines for four years, has more than enough hatred and recruits to terrorize the US for a generation.
Not exactly the clean, "fight them over there so we don't have to fight them over here" argument, eh?
(I am struck by the parallel that we are also confronting Iran over its nuclear program to prevent the Israelis from "confronting" Iran.)
Note: This post is not an endorsement of Clark's argument, just a presentation of a Democratic rationale for staying in Iraq.
Also, as Matteo pointed out in the comments, there's already ample evidence that "private citizens" in Saudi Arabia are funneling tens of millions to Iraqi Sunni groups, money that funds attacks on US Marines.