Assad has been a state sponsor of terrorism (as "Crows Nest" writes above), and there will be "blow-back" regardless of  what we do or don't do.  In addition, the role of the Muslim Brotherhood poses a serious question. 

Syria has become a prime example of the perils that often exist in making foreign policy: there are not any good, risk free choices, and decisions need to be taken in the face of considerable uncertainty.  In this case, Syria is already in the midst of a bloody civil war, others already are intervening (Hezbollah, Iran, Russia), and there are good reasons for choosing to act rather than abdicate.

Responding to Rob Long (#23). 

Good point.  The appetite for risk, or more broadly a willingness to adapt and be innovative matters greatly.  Over the years the US has shown remarkable flexibility.  This has proved invaluable in crises.  The question now is one of whether we still possess that capacity.  Paradoxically, the more serious the sense of crisis, the greater the likelihood that the needed steps will be taken.   I'm cautiously optimistic that this will prove to be the case, but it is by no means a certainty.  Note that the subtitle of my book is meant to signal this sense of contingency. It reads, Why the US is Not Destined to Decline, NOT Why The US Won't Decline.

Decisive action to cope with America's problems of national debt, deficit, and entitlements is critical.  Solutions and reforms are feasible, but the longer we wait, the more difficult they become. Winston Churchill once said that, "Americans can always be counted on to do the right thing...after they have exhausted all other possibilities.” There are reasons for cautious optimism about our ability to tackle these problems, but this requires leadership and resolve that are not yet evident.