Instead, we should focus on a minimalist definition of our interests in Iraq, which is to prevent a militant Sunni jihadist mini-state from emerging and allowing al-Qaeda to regroup. While withdrawing a substantial number of American troops from Iraq would probably tamp down the insurgency and should be done as soon as is possible, a significant force must remain in Iraq for many years to destroy al-Qaeda in Iraq.
I agree with Bergen that we should prevent such a bad thing from happening, but I disagree on the means.
Fortunately, the emergence of a "militant Sunni jihadist mini-state" is not as likely as Bergen thinks. Al-Qaeda in Iraq is actually fairly unpopular even in Al-Anbar and Diyala provinces, to the extent that the police chief in Baquba has said that they have been "driven out and finished off." What's more, General Casey is now openly admitting that "engagement with the resistance with a view toward decreasing violence and bring [sic] them in the political process" is a key component of its strategy (more speculation on that here from Marc Lynch). I would bet that the U.S. is demanding that insurgent groups show that they are willing to crush al-Qaeda in Iraq, and that they will not export violence once the U.S. leaves. We don't yet know if these negotiations are serious or merely a reprisal of previous failed efforts, but they present an intriguing alternative to Bergen's approach. Staying to fight al-Qaeda ourselves, in contrast, would prevent the broader Sunni Arab insurgency and tribal leaders from isolating and crushing al-Qaeda and entering the political game in earnest.
One more thing: can we get Osama, please?