Fred Kagan responds to Biden’s Washington Post op-ed here. This is his counter-argument to a key Biden contention:

We are not simply “squeezing the water balloon.” Violence is up in the Baghdad belts because U.S. and Iraqi forces have been aggressively attacking al Qaeda bases in those areas that have been funneling weapons and fighters into Baghdad. Naturally when we attack his critical bases and lines of communication, the enemy fights back. The U.S. command has responded by sending more force into this area to exploit initial successes, which have played a role in keeping the AQI (al Qaeda in Iraq) violence in Baghdad under control. Biden would do well to follow events more closely, and then he would see the interconnection between the Baghdad belts and the effort to secure Baghdad. He did not note, but might have, that violence has also increased in Diyala province as AQI fighters driven out of Baghdad and Anbar are seeking new bases. The U.S. command has responded by increasing forces in Diyala to fight the new threat, and is making progress. Ba’qubah has been partially cleared and operations against AQI bases there are continuing. But all this raises the question: Doesn’t Biden believe that we should be fighting al Qaeda? Most of his colleagues in the Democratic party say that that is the only interest we have in Iraq. It’s the interest that’s being pursued by our operations in the Baghdad belts, Diyala, and Baghdad itself. Attacking the enemy increases violence in war. Indeed, it’s often the only way to attain an important objective like defeating al Qaeda, an objective that seems to be, for the first time, coming nearer to our grasp.