I thought that as well, initially, but think about it. Although Obama ran on ending the Iraq war, he allowed it to end on Bush's timetable. He surged troops in Afghanistan when needed. And he attacked Libya, assisting the rebels and our European allies in overthrowing Qadaffi. Does he really need to establish his/our willingness to strike other nations right now?
Conventional wisdom is that if you make a threat and don't follow through, you lose credibility for future threats. As a principle that is undeniably true, but increasingly I'm thinking that in this specific instance we actually lose little credibility by not striking.
And morally the situation is so muddled that I don't think anyone can predict ahead of time whether the moral high ground is to strike at Assad for using chemical weapons, both because it may well turn out this was a rebel operation (or accident) and because the number of people killed by WMD is probably 1% or 2% of the total civilian death toll.