I see four core American interests in Iraq that cannot be abandoned. There must be no Afghan-like Al Qaeda takeover of wide areas. There must be no genocide (say a Shiite sweep against Sunnis). There must be no regional conflagration (for example, a Turkish invasion). And there must be no return to the old order (murderous Stalinist dictatorship).
To ensure this, the United States must keep a military presence in Iraq for the foreseeable future. The size of this deterrent force is up for debate, but 50,000 soldiers, or 105,000 less than today, is one talked-about figure. The timing of the drawdown will have to be discussed with Nuri Kamal al-Maliki, the Iraqi prime minister, but it should begin soon after September.
Pulling out a lot of troops is the only way to increase pressure on Maliki to make the political compromises - on distribution of oil revenue, the constitution and de-Baathification - that will give Iraq some long-term chance of cohering. That chance will be increased if, as the United States steps down, the United Nations steps up.
What drives me nuts about this residual force stuff, aside from how arbitrary it is, is that there's never any thought to exactly what these 50,000 should do. Basically, as the violence rages around them they're supposed to sit there to ensure that... there isn't even more violence raging around them. But it isn't really enough people to actually intervene, especially given that not even close to that many would be combat troops.
As for that throwaway line at the end (and it's the only time in the whole piece where the UN is even mentioned), wtf? Just what is the UN supposed to do?