From Barrons [it may also be found here]:
When [President Barack] Obama proclaimed his red line on Syria and chemical weapons, he assumed the issue would not come up. He made a gesture to those in his administration who believe that the United States has a moral obligation to put an end to brutality. He also made a gesture to those who don’t want to go to war again. It was one of those smart moves that can blow up in a president’s face when it turns out his assumption was wrong. Whether al Assad did launch the attacks, whether the insurgents did, or whether someone faked them doesn’t matter. Unless Obama can get overwhelming, indisputable proof that al Assad did not — and that isn’t going to happen — Obama will either have to act on the red line principle or be shown to be one who bluffs. The incredible complexity of intervening in a civil war without becoming bogged down makes the process even more baffling.
Obama now faces the second time in his presidency when war was an option. The first was Libya. The tyrant is now dead, and what followed is not pretty. And Libya was easy compared to Syria. Now, the president must intervene to maintain his credibility. But there is no political support in the United States for intervention. He must take military action, but not one that would cause the United States to appear brutish. He must depose al Assad, but not replace him with his opponents. He never thought al Assad would be so reckless. Despite whether al Assad actually was, the consensus is that he was. That’s the hand the president has to play, so it’s hard to see how he avoids military action and retains credibility. It is also hard to see how he takes military action without a political revolt against him if it goes wrong, which it usually does.