US military involvement might start as a limited campaign (a no-fly zone, air strikes, or other direct military tactics – short of boots on the ground) meant to stop the bloodshed, remove Bashar al-Assad, and secure a large chemical weapons arsenal. But it could end up with severe unintended consequences: a regional war, more bloodshed, a power vacuum that could favor jihadist elements, and weapons of mass destruction that end up in terrorist hands.
Obama also holds concerns that direct military engagement in Syria might upset his carefully crafted strategy on Iran, which he thinks is working. Obama strongly believes that his administration has achieved notable successes on the Iranian nuclear front – an effective sanctions regime that is hurting Iran’s economy and an international coalition that seems equally committed to the goal of convincing Iran to scale down its nuclear aspirations.
Neither of these developments is guaranteed to change the nuclear calculus of the Iranian leadership, but Obama likely judges that too much effort has been put into the Iranian issue over the past four years to see it ruined now by fishing in Syrian troubled waters. Syria is Iran’s only ally in the region, and US military action against the Assad regime is likely to trigger overt intervention by Iran, which could escalate into a US-Iran military conflict. Obama certainly does not want to be forced to go to war against Iran because of Syria.