He'll be telling the nation tomorrow night. In the meantime, Andrew Sullivan recommends reading the transcript of Obama's press conference at the G20 summit in Russia, which includes this:

My goal is to maintain the international norm on banning chemical weapons. I want that enforcement to be real. I want it to be serious. I want people to understand that gassing innocent people, delivering chemical weapons against children is not something we do. It’s prohibited in active wars between countries. We certainly don’t do it against kids. And we’ve got to stand up for that principle.

Sullivan remains unconvinced, and so does Congress.