I tend to agree with IPride that this was a good, if not a master move, by Obama. He does not have the allies out there (other than the French and to a much lesser extent Turkey and some of the Arab league) and he wanted to make sure that he at least has bipartisan support within the US. This is a HUGE decision which can have serious repercussions not just for Democrats in the US but possibly for US foreign policy in the ME and the entire region for years to come.
He left himself between a rock and a hard place with his "red line" comment and interestingly enough, that was made after Bibi's red line comments on Iran. One way or another, this is the blue print for how to deal with Iran, which is a much more complex situation than any of the wars the US has been involved with in the ME. He either has to go through with this and set a precedence, or the only way to back down at this point was if he didn't get support from congress.
He also realizes that this would be buying time while applying pressure at the highest levels and that seems to have paid off already. We've already had a major defection in Syria and Putin is not saying that he might drop support for Assad if they're given more evidence. All in all, he may come on top in this battle without firing a single cruise missile.