EVANSTON, Ill. — 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. Secretary of State John Kerry seemed to do just that on Monday, when he said of the use of chemical weapons, “This international norm cannot be violated without consequences.” His use of the word “norm,” instead of “law,” is telling.

Syria is a party to neither the Biological Weapons Convention of 1972 nor the Chemical Weapons Convention of 1993, and even if it were, the treaties rely on the United Nations Security Council to enforce them — a major flaw. Syria is a party to the Geneva Protocol, a 1925 treaty that bans the use of toxic gases in wars. But this treaty was designed after World War I with international war in mind, not internal conflicts.

What about the claim that, treaties aside, chemical weapons are inherently prohibited? While some acts — genocide, slavery and piracy — are considered unlawful regardless of treaties, chemical weapons are not yet in this category. As many as 10 countries have stocks of chemical weapons today, with the largest held by Russia and by the United States. Both countries are slowly destroying their stockpiles, but missed what was supposed to be a final deadline last year for doing so.