However, given the fact that any intervention by the Obama administration is likely to be tentative and halting, rather than an overwhelming show of military force, it is not likely to end Syria’s civil war or intimidate Iran’s rulers.

The sort of intervention needed to bring about a decisive rebel victory would require more than no-fly zones and arms. It would mean disabling Mr. Assad’s air power and putting boots on the ground. America would have to take the lead in organizing a regional military force blessed by the Arab League and supported by its own intelligence assets and Special Forces. After that would come the task of reconstituting Syria and mediating its sectarian conflicts. As the war in Iraq painfully demonstrated, refashioning national institutions from the debris of a civil war can be more taxing than the original military intervention.

Because it would take all of this to oust Mr. Assad and end the violence, America must accept the need for a robust intervention. There is no easy solution or middle ground. Moreover, rather than intimidating Iran, a less-than-decisive American intervention in Syria would do the opposite. It would convince Iran’s leaders that America doesn’t have an appetite for fighting a major war in the region.