Digby: Are They Necessary?

Atrios brings up something I've been wondering about. It's assumed by most of us that one of the reasons we are in Iraq is because we want to establish permanent military bases there -- presumably because we had to remove the "temporary" ones we had in Saudi Arabia (at bin Laden's rather dramatic request on 9/11.)

Maybe it's just because I don't fully understand the military dimension but why do we need permanent bases in either Saudi Arabia or Iraq?