YOU CAN'T ALWAYS GET WHAT YOU WANT. Ezra, I really, really don't like disagreeing with Peter Bergen on al-Qaeda. You generally should be extremely wary of telling a guy who interviewed bin Laden that he's off-base. But, dude, you asked. Thanks, Ez, you're a good friend.
Let me start by saying that Peter is 100 percent right that bin Laden & co. want to take over Iraq. But, to expand a bit on a point that Blake made, "want" and "can" are two different things. Peter may be a bit skewed by his deep knowledge of Afghanistan here. It was pretty easy for al-Qaeda to adapt to a post-Soviet Afghanistan. From the evidence so far, that's really not the case in Iraq: not only do the Iraqi Sunnis really dislike al-Q, but Anbar province has even assembled its own anti-Qaeda death squad. There's only one thing that could stop the Sunnis from fighting al-Qaeda: their greater desire to fight us instead.
There's also a Machiavellian aspect here. To be extremely callous (given that we're talking about human life here), it's not exactly against U.S. interests to let Iraqi Sunni (and Shiite, for that matter) fury at al-Qaeda take its course. One of the most valuable, if underappreciated, weapons in our war-on-terrorism arsenal is the recognition among Muslims that no matter how bad they think we are, bin Laden doesn't exactly offer them a bright future. We would definitely reap the blame -- and we should -- for the ravages of the Iraqi civil war, but from the perspective of fighting al-Qaeda, as long as the bodies of the jihadis pile up, that's probably tolerable.