At this stage, arming the opposition would not serve U.S. interests. A military escalation would probably invite chemical attacks by Assad’s forces. Unless the United States can ensure that arms supplies would be transferred only to like-minded factions of the opposition — and it cannot — the risk of sectarian tensions spilling across the region will increase. Disintegration of the Syrian state will threaten its territorial integrity as well as Iraq’s, particularly given each country’s restive Kurdish populations. …

The most obvious options to curtail the bloodshed in the near term could have problematic consequences down the line. Strikes against Syrian air defenses and the enforcement of a no-fly zone would boost the extremists in the opposition. A tougher line against Russia and Iran, which are stoking the fighting, could preclude their buy-in for a longer-term understanding to keep the conflict from spreading.

A U.S.-Russia deal on the relatively low-hanging fruit of disarmament could generate momentum for a broader political settlement. But if Moscow will not advance even an agenda as mutually beneficial as chemical weapons disarmament, the United States will need to consider next steps on the assumption that Russian assistance will not be forthcoming until battlefield realities change decisively.