There was too much at stake with possibly the Chinese and also the Soviet Union. You may recall that during the bombing runs certain parts of Haiphong Harbor were off limits because Soviet Ships were in the Harbor and we were afraid to hit them. This was not a declared war but a conflict and the Soviets and Chinese were supplying all types of weapons. Just like Korea where we did not want to attack over a certain parallel because of fear the Chinese would enter the war (some say they did anyway).
There is no way we would have won Vietnam as we had really no dog in the hunt. Just like there is no way we will win in Iraq. In Vietnam we needed the Vietnamese to fight their own battles which took us a long time to accomplish - it was called Vietnamization.
In Iraq we need the Iraqi's to fight their own battles but against who? Of all the prisoners we have captured in Iraq only 600 are foreigners. Who is the enemy?
I suspect that if we leave then the foreigners will leave as well - no more Americans to kill. The population will then engage in a civil war if they are not happy with the constitution - which is the issue. The Sunni's will fight on because I just don't see them winning with the Shiites population and the history of the past abuses. The Kurds just want their piece up North where Turkey will eliminate them if they break away from Iraq (so we could have a role here protecting them from Turkey). Of course the Kurds will demand the oil fields of the North as well. Iran will not want anything but an Islamic country. It's a mess and like Vietnam will take many more American deaths before our politicians will need to find "Peace with Honor" again.
I say tell the Iraqi's we will leave right after they agree on a constitution which has to be agreed to within six months. If not then Bye Bye. If they don't want to govern themselves then it's not worth it.
William Hung is to American Idol as Sarah Palin is to the Vice Presidency