he didn't need Congressional authority - if he didn't for Libya, why would he need it for Syria?
All he had to do was call it "kinetic military action"- like Libya.
Look at some articles from the time, Congress demanded he not take action without a vote. They were against him taking action, he was going to lose the vote. They were speaking out against him. We don't attack a foreign nation without unified agreement. It simply does not happen. Libya was about as controversial as it gets, and that was iffy. Yes, Obama could have done it without Congressional approval, but he was already being undercut by his own Congress, the effort would have failed so he pulled back.