I'd like to say that Tedrbr still had it right in his previous posts.
The U.S. has been keeping Plans on scenarios in the middle east since Large deposits of oil were found there. During the cold war, we had plans to counter the Soviets in case they intervened or invaded. Why do you think we've kept such close ties to the Saud's this whole time?
You can't really draw parallels between this and Vietnam on a political level...It's not simply about keeping a country aligned with you politically.
This time, it's a(n) known source of oil, that's what we need for our economy. You guys all know, that if we get cut off of oil, We have massive problems in the U.S. I'm sure that we will never be completely out of the middle east, now that we're there.