I'm wondering why, exactly, did Obama put the Syria issue before Congress. This guy has killed all sorts of people, including American citizens. He's invaded countries, and even in his speech about punting this to Capitol Hill, he states he has all the authority he needs to bomb Syria all by his little self.
What's the deal? Well the timeline of seven countries in five years doesn't quite match up. It has been longer than five years. We know there is a plan. The people interested in executing the plan are very powerful, but not necessarily Obama's buddies. Interdepartmental rivalries have been suggested before- and in some case, like foreign policy, things can get so weird, that you've got to wonder.
I can't help but think this vote gives Obama a way to officially support action in Syria, while surreptiously making it very unlikely. It has the additional value of displaying his value to whatever powerful forces are pushing for war. Essentially, they'll see they don't have the option of weakening him and/or trying to push their agenda via someone else. There is no one else, and they are just going to have to wait longer, apparently.
I don't think he'd care much about American popular opinion, either. If he actually wants to bomb Syria, he just will. He's either just buying time, or doesn't want to do it. He is in his second term and doesn't need to worry about the popularity of what he is doing.