>Given the millions who were killed when the U.S. pulled out of Vietnam, can the U.S. ever pull out of Iraq?

I'm not so sure this would happen with Iraq. Vietnam at the point of the US pullout had essentially been 2 countries for 20 years. The US entered the war during hostilities and left the war during hostilities. The civil war was going to continue with or without a western power there.

The violence in Iraq, at least at first, was directed at the occupying armies. I think if the US/UK stay much longer the insurgents will become entrenched like they did in Afghanistan. A partition of Iraq into its historical 3 parts might make the most sense. The kurds, shia and sunni could then live in peace.

The Bush administration failed to understand the symbolism of European troops occupying Baghdad. This would have been liking sending British peacekeepers during the American civil war.