With the industrial revolution and advances in chemistry, many nations agreed in the Hague Convention of 1899 not to use “projectiles the sole objective of which is the diffusion of asphyxiating or deleterious gases,” which were not then widely understood. There was a follow-on agreement in 1907, but World War I proved just how hollow that effort was.

There have been only a few known instances of poison gas being used since 1925, and in each case the perpetrator never openly admitted it. In the first two cases, gas was used by authoritarian regimes against those they considered lesser races. In 1935-36, Mussolini used several hundred tons of mustard gas in Abyssinia, now Ethiopia, and in 1940-41, the Japanese used chemical and biological weapons widely in China, where unexploded poison gas shells are still being dug up at the expense of the Japanese government.

François Heisbourg, a special adviser to the Foundation for Strategic Research in Paris, argued that one reason Japan stopped the use of chemical weapons, while then denying their use, is that President Franklin D. Roosevelt stepped in and, in quiet diplomacy, “told the Japanese that we knew of the use and that there would be consequences.”

In 1965-67, President Gamal Abdel Nasser of Egypt ordered intermittent use of chemical weapons in the course of a long and disastrous war in Yemen, and the American use of Agent Orange in Vietnam was widely criticized, but it was legally considered a defoliant, despite its impact on human health.

It was only in the Iran-Iraq war of 1980-88, started by Iraq after the Islamic Revolution in Iran, that chemical weapons were again used in large amounts, and by the Iraqi leader Saddam Hussein against Iranian forces and his own Kurds. The Iraqis used both first- and second-generation nerve gases to blunt Iranian offensives in southern Iraq and forestall defeat. Given American and Western unease with Iran’s revolution, there was little public outrage as Muslims used poison on other Muslims.

The world’s indifference altered sharply, however, in March 1988, when Mr. Hussein killed between 3,200 and 5,000 Kurds around the town of Halabja and injured thousands more, most of them civilians, some of whom died later from complications.

The Halabja killings, considered the largest chemical warfare attack ever directed at civilians, led to the Chemical Weapons Convention of 1993, in force since 1997, which bans not just the use but also the possession, manufacture and transfer of chemical weapons. It has since been signed and ratified by 189 states, according to the Organization for the Prohibition of Chemical Weapons, which carries out the treaty. Syria is among only five states — with North Korea, South Sudan, Angola and Egypt — that have neither signed nor ratified it.