It's very hard to get out of a war, even a war that most know that you are losing. If you are a superpower, and your opponent is weak mini-state, the difficulty of extraction is greatly increased. An awful lot of people are certain to have a stake in continuing the war long after it becomes clear that the original goals can't be achieved, or can't be achieved at an acceptable cost. Foremost among those are the idiots who got you into the war in the first place, in our case, the President, the Congress, and the Neocon cabal.
They aren't the only ones of course. For many, a war is a great moneymaking opportunity. For a partially outsourced war like Iraq, the opportunities are multiplied. All those who supply weapons, food, fuel, equipment, and contract mercenaries are making a killing, and they don't want the war to stop.
The soldiers, though, are a key. In Vietnam, and in the Soviet disaster in Afghanistan, the soldiers were mostly draftees with little investment in the war or their military units, so they were relatively quick to see the disaster unfolding. The professionals in Iraq are different. To a far greater degree, they are committed to the military and especially to their own units. Giving up is seen as a betrayal of their sacrifice and the sacrifice of their fellow soldiers. So long as they see any hint of progress, it is very difficult for a professional soldier to concede defeat.
Finally, there is us, the mass of citizenry without a direct stake in the war. If our own husbands, wives, and children aren't at stake, the wars depredations don't hurt so much, and nobody wants to admit to being a loser. Also, we are easy prey to the scare mongers who promise horrible things will occur if "the mission fails."
What is needed is an Eisenhower or a Rabin, a man of undoubted military credibility who will say - "I will end the war." There is no such person now alive.