going on the experiences of late, in Egypt and those other countries i have listed, you knock out the bad guy to be replaced with worse, the rebels are not going to accept our intervention, they just want to kill the bad guy, then install their own brand of government, amongst the rebels are many hard line Islamists, who the west has been fighting against and dying in their thousands, not to mention the collatoral damage of the people in the middle, if the US steps in, who will get the back of Assad, we know who, and that does not bear thinking about.
This is a crisis but not one we should take any hardline approach to, nor the US they do not want to see more bodies coming home to US soil, it would be massively unpopular with the American people, as it would here if we go in