Michael Doran and Salman Shaikh do an impressive job of demanding that the U.S. arm the Syrian opposition without bothering to explain why the U.S. has a stake in aiding their victory:

It is important to remember that arming the FSA is a political act. The most important decision of all is simply to provide lethal assistance. The goal of the operation is to build a force on the ground that is more likely to respect American interests and that is committed to building a nonsectarian, stable Syria. Even the provision of light weaponry would be a good start to this project.

It’s often taken for granted that the U.S. should want to support the Syrian opposition directly for one of two main reasons: 1) to weaken Iranian regional influence; 2) to have a better chance to “shape” post-Assad Syria. Neither of these reasons is any good. On closer inspection, the argument for arming Syrian rebels doesn’t take into account the full costs of such action, and it treats intervention in the Syrian conflict as if it would have no effects on other U.S. policies or goals elsewhere.

It is very unlikely that sending in arms shipments will give the U.S. much real influence over the politics of a post-Assad Syria, since the proxies that the U.S. arms will pursue their own goals regardless of what Washington wants. It is likely that the U.S. will be directly supporting armed groups that use weapons supplied by our government to exact retribution on prisoners and civilian populations on the other side of the conflict. The U.S. has no interest in increasing regional instability even more, but that is what arming the opposition would achieve.

The idea of waging a proxy war against Iran in Syria might make sense to Iran hawks, but the truth is that the U.S. has no good reason to contribute to a Syrian civil war in the hopes of doing indirect damage to Iran. Doing links Syria’s conflict to U.S.-Iranian antagonism in a way that would make both worse, and it would make it even harder for the parties to the conflict to reach a point where they are willing to accept a negotiated settlement. Once the U.S. committed itself to directly supporting the Syrian opposition, the administration would be under increasing pressure not to let “our” side lose, which would in turn open the door to direct military intervention. The case for arming Syrian rebels still doesn’t hold up under scrutiny, and it is doubtful that it ever will.