Peter Suderman in Reason's Hit and Run blog:
1. If the rebels win, it’s bad news for the U.S. Assad is no friend to the U.S. But neither are the rebel groups leading the charge against the Syrian dictator. Indeed, many of the rebel factions have strong ties to Al-Qeada. If the rebels successfully oust Assad, it’s entirely possible that they will attempt to set up a new regime that is intensely hostile to the United States. Intervention on the side of the rebels would also complicate America's already-fraught relationship with Russia, which is close with the Assad regime.
2. If Assad wins, it’s bad news for the U.S. Especially if the U.S. is seen to have openly sided with the rebels. A win for Assad is a win for anti-American forces Iran, which would see its influence in the region strengthened. It’s also a win for Hezbollah, which is closely linked with Iranian extremists. With no good option, then, the U.S. is better off staying out of the conflict entirely.
3. It’s far from certain that any “limited” actions would actually be effective. Most of the talk right now revolves around the possibility of limited cruise missile strikes and/or no-fly zone enforcement. But as the Chairman of the Joint Chiefs of Staff Martin Dempsey told NPR last month, the possible results of enforcing a no-fly zone could “include the loss of U.S. aircraft, which would require us to insert personnel recovery forces. It may also fail to reduce the violence or shift the momentum because the regime relies overwhelmingly on surface fires — mortars, artillery, and missiles.”