I mostly agree with Matt Yglesias, as against Jacob Weisberg: the problem isn’t that we’re losing in Iraq; the problem is that the world, the Middle East, Iraq, and the United States are all less safe as a result of our invasion, even though we won.
But if we remove substantially all of our forces from Iraq without having pacified the country, that will certainly look like defeat. Or at least that’s what it’s usually called when an army retreats without having achieved its objective. We can rely on al-Qaeda to make the most of the plausible claim that jihadists pushed the Soviets out of Afghanistan and the Americans out of Iraq (having previously forced us out of Saudi Arabia).
No, that’s not military defeat, Vietnam-style. But it would be a big blow to our (already tattered) prestige, and a big boost to that of our enemies. That’s why people like Wesley Clark, who are smart and knowledgeable about this stuff and (unlike me) were smart enough and knowledgable enough to get the right answer to the question “Should we invade Iraq?” back when the decision had to be made, don’t want us to pull out now if there’s still any chance of salvaging a non-horrible outcome.