An even dozen commentators weigh in on whether the US ought to get involved in Syria.
I agree with most of them that going in is bad policy. A US claim to self-defence would be laughable. Neither is there a US interest in supporting the ruling Baathists or the Al Qaeda opposition. There is no prospect for a good outcome*, even if everything goes according to plan. There are many ways that things could go badly: the humiliation of the US appearing to lose to Assad, alienating the US from other ME states, strengthening Russian ties, strengthening Iranian ties, starting a larger war. Even if they to a successful and clean operation that allows the opposition to take down Assad more easily, US interests will have been harmed.
The only valid reason for American involvement is this. Barack Obama said that if Assad used chemical weapons, there would be consequences. Assad appears to have used chemical weapons on a large scale. How much is the president’s word worth?
*(There is one slim prospect for a good outcome but it is very Machiavellian. If Assad seems poised to win, the US could weaken Assad so the rebels can fight longer, thus weakening both Baathist and Al Qaeda, possibly offering innocents in and out of Syria some relief, and possibly causing the ME to tire of war.)