Cameron was the chief cheerleader that was pushing for war. The US was supposed to reluctantly join the call to partner their British ally. Unfortunately Cameron had his wings clipped so the administration have to sell it to the American public directly.
When in the summer that it was evident that the rebels were going to lose, a decission for the west to intervene on their side became inevitable. The US, British, Turkey and the French have been building up their navys and men in the region for several weeks.
When they were ready to go, by a strange coincidence, Assad used chemical weapons, therefore providing the started trigger for the war to start.
The US, France and Turkey are welcome to fight this one without us. Unfortunately, when they want the UN to go in and hold the peace, guess who will be first in the queue to send our troops?