My unredeemed, gut-instinct: let them.
Tom's got a great post on the ME today, The Big Bang turned in on itself. Let's see if I can excerpt it without just copying the whole thing.
Can't remember which book I wrote it in, but I've maintained for a while now that Iraq works either way: goes well, it scares the region toward change (what we saw deep into 2005); goes badly (since April 2004), it also scares the region into change by raising the specter of spillover (locate the fighters where the fight really is).
In this rather Machiavellian view, Bush gives Osama what he wants (lotsa conflict spreading in the region) but should be smart enough to fear (his cause gets lost in classic Shiia-Sunni fighting). The be-kind-to-my-Commander-in-Chief instincts within me are definitely attracted to anything that, as Bret Stephens (I think) once wrote, essentially "speeds the killing." There are so many unredeemables in the region that simply need to be killed off by history, no matter who pulls the trigger.
Big point: the killing is exactly where it should be, as in over there and not over here. Pull our troops back just so, and there's no antiwar anything in the U.S., because we routinely ignore big wars overseas so long as Americans aren't involved (we are an incredibly myopic nation in that regard--save our military).
I have no desire to side with Sunni versus Shiia, and to conflate al-Qaeda with radical Shiia. I see no reason whatsoever to have a dog in this fight, just to get the fight club rolling.
There are three fights we want to speed the killing (or just the threat of killing) on:
1) Israel versus Iran (the nukes issue must be used to create a strategic relationship there, naturally bad and scary at first, just like ours with the Sovs was);
2) House of Saud (it ain't a country) versus Iran (which definitely is);
3) House of Saud and al-Qaeda.
That's why to me, the trifurcation of Iraq is not a bad deal whatsoever, but an inevitable one (following the Yugo[slavia] example) that we need to exploit to the Big Bang's purposes. The Kurds are saved (and become the place where our troops should be storehoused). The Shiia in Iraq become an effective conduit of change into Persian Shiia Iran (it plays Walesa's Poland to Brezhnev's USSR). The House of Saud is finally forced to grow up and become a real country. Al Qaeda's real dreams of toppling the House of Saud are effectively crushed through that regime's forced rehabilitation.
Bush proves time and time again that he does not listen to his generals, so do not count on that common sense restraining him or Cheney.