Maxspeak asks a bunch of good questions, and comes up with this answer:
Maybe I'm fighting the last war (or several wars back), but I sense a familiar dynamic. The troops will be the peace movement. The troops will want to come home, and they will be hard to refuse. I went to college a right-wing Democrat, supportive of the war (though god knows not enough to actually enlist). Returning veterans did a lot to turn me around. The weight of the lost cause grew on all who were deluded into envisioning an idealistic and successful U.S. role in Vietnam. In Vietnam, it was never a case of the U.S. military being unable to acquit itself well in actual battle. It was the failure to construct a loyal society out of fundamentally disaffected masses of people. It was good money after bad, each additional life lost more pointless than the preceding.

So far there seems to be no difference in Iraq. You have two coherent national groups -- Kurds and Shi'a -- with no desire to be ruled by U.S. surrogates, and no need to knuckle under to the U.S., and you have the Saddamist-saturated Sunni.

I could be wrong, but I see the U.S. in a hole, and still digging.

Lots of interesting comments to that post as well.