Thus was launched “the chemist's war,” a scramble to meet horror with horror. By the end of World War I, more than 90,000 soldiers had been killed by poison gas — many after weeks of agony. A million more men had been blinded or injured for life.

Of course, chemical weapons accounted for a small fraction of the war's 17 million deaths. But world leaders reacted quite viscerally to their use. Perhaps, chemical weapon historian Jonathan Tucker observed in his book “War of Nerves,” there is an “innate human aversion to poisonous substances.” Or else, he posited, their use seemed cheap somehow, a “duplicitous use of poison by the weak to defeat the strong without a fair physical fight.”

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Other researchers have suggested that chemical gas inflicts a particular kind of psychological torture. “People fear gas in a way they don’t fear conventional weapons. I don’t think it’s based on any rational analysis of battlefield statistics,” Andrew Ede, a science historian, told Chemical and Engineering News. “I think it’s just based on the idea of what it would personally be like to suffer through a gas attack. The public perception is that it is evil and unsportsmanlike.”

By 1925, the League of Nations had drafted a treaty to ban the use of such gas during war. Most countries signed. Even those who didn't adhere to the general principle: that chemical weapons have no place in war. (Though over the next fifty years, many countries -- including the United States and the Soviet Union — would build out their own stockpiles, just in case.)

Over the past century, a handful of rogue trouble spots — including Iraq, Iran and Syria — have deployed chemical weapons. And while this tactic often brings swift, international condemnation, there's rarely much lasting punishment.

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In 1936, for example, the Italian dictator Benito Mussolini dropped mustard-gas bombs in Ethiopia in an attempt to destroy Emperor Haile Selassie’s army. Countries grumbled, but no international action was taken. Egypt used mustard gas and a nerve agent in Yemen to support a coup d’état against the Yemeni monarchy, to little notice.

And in 1985, Iraq used an array of chemical weapons against Iran, with then-President Ronald Reagan's tacit approval. Saddam Hussein repeatedly attacked Iranian forces with sarin, killing more than 20,000. Thousands more were injured. The next year, he attacked his country's Kurdish minority with gas, killing around 5,000 civilians.

At the time, Hussein was a U.S. ally; his actions went without punishment. “The muted response by the international community — including the United States — probably encouraged Saddam to continue using chemical weapons in that war, and also encouraged other countries in the region to develop their own,” Daryl Kimball, executive director of the Washington-based Arms Control Association, told my colleagues at The Washington Post.

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Syria, too, has used its stockpile of chemical weapons (“the poor man's nuclear arsenal”) to ward off potential enemies. In the 1970s and '80s, Syria lost three wars to Israel. Soon after, it began producing its own chemical weapons to counter Israel's vastly superior military and alleged nuclear arsenal. Today, President Bashar al-Assad probably controls about 1,000 tons of chemical weapons, stashed in 50 facilities. A former Pentagon official called the country a chemical weapons “superpower.”

Of course, Israel is not the besieged Assad's only enemy. In 2013, years into Assad's power struggle with rebel forces, the government launched a brutal sarin attack on the outskirts of Damascus. According to the White House, Syrian officials killed nearly 1,500 people, including more than 400 children.

The gruesome photos seemed to provoke President Barack Obama to intervene. “What message will we send if a dictator can gas hundreds of children to death in plain sight and pay no price?” he asked at the time. “What’s the purpose of the international system that we’ve built if a prohibition on the use of chemical weapons that has been agreed to by the governments of 98 percent of the world’s people and approved overwhelmingly by the Congress of the United States is not enforced?”

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The specter of invasion spurred Assad to agree with a joint American-Russian proposal that he eliminate his country's chemical weapons program and join a 1997 chemical weapon ban treaty. But the Organization for the Prohibition of Chemical Weapons, an international monitoring body, found evidence that Assad used chlorine gas against opposition groups in 2014 and 2015. (The government has denied those reports, arguing that it is being framed by the Islamic State.)

And this week, a toxic chemical attack killed at least 58 people in the northern Syria province of Idlib. As Washington Post correspondent Louisa Loveluck reported: “Airstrikes on the northwestern town of Sheikh Khanoun began just after daybreak, delivering an unidentified chemical agent that killed at least 58 people and filled clinics across the area with patients foaming at the mouth or struggling to breathe.” Photographs and online video show children and older adults gasping for breath. In one image, a group of 10 children lie lined up on the ground under a quilt. Rescue workers said people collapsed outdoors, in large numbers; several people were sickened simply by coming into contact with the victims.

Locals said the attack was delivered by a government airstrike; a subsequent airstrike targeted one of the clinics treating victims. (The area's largest hospital had been severely damaged two days earlier by an attack).

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Once again, the world has called for action against Assad. Europe’s top diplomat said the Syrian government bore “primary responsibility.” French Foreign Minister Jean-Marc Ayrault condemned the “disgusting act” and demanded an emergency meeting of the U.N. Security Council. The Trump administration called the assault “reprehensible” and “intolerable.” Sen. John McCain (R-Ariz.) challenged the Trump administration to take action against Assad’s “war crimes.”