The latest atrocities in the Syrian civil war, which has killed more than 100,000 people, demand an urgent response to deter further massacres and to punish President Bashar al-Assad. But there is widespread confusion over the legal basis for the use of force in these terrible circumstances. As a legal matter, the Syrian government’s use of chemical weapons does not automatically justify armed intervention by the United States. There are moral reasons for disregarding the law, and I believe the Obama administration should intervene in Syria. But it should not pretend that there is a legal justification in existing law....