U.S. inaction in Syria could be far more costly than intervention

 'The one advantage of inaction seems to be the ability to disclaim responsibility: We didn’t break it, so we don’t own it. Even that benefit, however, may prove transient. Already the United States is the largest donor of refugee aid. As misery spreads and anti-American radicals plant roots, the Obama administration, or its successor, may find that the costs of non-involvement far exceed those that would have come with timely and measured intervention.'

I don't personally see the risk in option (a), Arm the Moderate Rebels. And a bit of extremism in defence of liberty is no vice. Barry Goldwater said that. He was a very conservative US politician.