The U.S. has no legal basis for its action in Syria, but that won’t stop us from going in anyhow. - Slate Magazine
The Geneva Protocol of 1925 (which Syria ratified) and the Chemical Weapons Convention of 1993 (which Syria has not ratified) ban the use of chemical weapons, but do not authorize countries to attack other countries that violate these treaties. The United States has no more authority to attack Syria for violating these treaties than it does to bomb Europe for giving import preferences to Caribbean banana producers in violation of international trade law. At one time, countries could use military force as “countermeasures” against treaty violators, but only against violators that harmed the country in question—and Syria has not used chemical weapons against the United States—but in any event, that rule has been superseded by the U.N. Charter.