The Obama administration argues that the same criteria don’t apply in Syria. The killing there has been undeniably terrible, with estimates running as high as 13,000 dead since the uprising began. But in Libya, Obama was acting specifically in response to the imminent storming by Gaddafi’s forces of Benghazi, a city of 700,000, where Obama predicted an indiscriminate massacre. Meanwhile, there’s a consensus that military action in Syria would be far riskier than it was in Libya, thanks to Syria’s more sophisticated military forces and air defenses.
The Syrian opposition is also even more splintered and inchoate than were the Libyan rebels, and Assad is more popular than was Gaddafi. International bodies like the United Nations and Arab League haven’t called for intervention, and Syria’s ties to Russia–and Iran–also introduce all sorts of strategic complications that didn’t exist in the case of Libya, which had few reliable friends and little geo-strategic import. Merely arming the opposition, as Romney has proposed to do, eliminates the risk to American soldiers, but the other concerns still apply.