Over the last 20 years there have been numerous cases where various governments have probably or definitely used chemical weapons against citizens and we've stood by and done nothing. Nothing. Didn't even lodge an objection.
Added to that, there is a chance that the Assad government didn't do this at all. There have been stories floating around for years now that one way to get us involved in Syria was for someone to do something just like this. We've been fooled before. I want absolute proof from another government, not ours. They've lost all credibility. And not one of those governments that asks "how high?" every time we say jump.
But now this.
I'm reminded of General Wesley Clark's remarks about what happened after 9/11, when he was told first that Bush had decided to invade Iraq even it though it had nothing to do with terrorism. Why? The general that was telling him about the plan said, “I guess it’s like we don’t know what to do about terrorists, but we’ve got a good military and we can take down governments.” And he said, “I guess if the only tool you have is a hammer, every problem has to look like a nail.” Then a few weeks later he was told that the plan had been expanded to "take out" seven countries in five years: Iraq, Syria, Lebanon, Libya, Somalia, Sudan and, finally, Iran.
And he's gone on to tell that story over and over again to everyone that would listen.
Obama doesn't know what to do about the domestic/international spying debacle he's got himself into, he doesn't know what to do about the economy (I mean, Jeebus, Larry Summers for the Fed? Is he insane?), and all of the other distractions he's tried haven't worked. His legacy is probably shot, and the best he can hope for is to pull a draw. He doesn't know what else to do, but we've got a good military and we can take down governments.