Ian Hurd, an Associate Professor of Political Science at Northwestern University, writing at preeminent international law blog Opinio Juris:

It is well known that the [U.N.] Charter forbids the use of force except as self-defense or as sanctioned by the UN Security Council. Everything else amounts to aggression and is illegal.

The issue of whether and how the U.S. and/or the rest of the world should react to the use of chemical weapons in Syria is open to debate on ethical, moral, political, and practical levels.

But it is not open to any debate from an international law perspective: the U.S. proposal, whether approved and implemented by President Obama or the Congress to which he has deferred on the decision, is prohibited generally by international law and specifically by the United Nations charter.