Chemical weapons have a long, lethal history. Accounts of ancient chemical warfare, including the use of poisonous smoke and arrows, date back as far as the 12th century, B.C.E. And while the use of "asphyxiating, poisonous or other gases" or "bacterial methods of war" was banned at the 1925 Geneva Conference, under the auspices of the League of Nations, such weapons have emerged in conflicts around the world.

In 2009, British archaeologists uncovered the oldest known evidence of a chemical attack at Dura-Europos, an ancient Roman city whose ruins are in the eastern part of modern-day Syria. There, 20 Roman soldiers died in a battle in 256 C.E. after inhaling poison gas from a stove that pumped fumes from burning sulfur crystals and bitumen into tunnels where they were barricaded. Ancient Chinese are said to have used bellows to pump arsenic fumes in the direction of their opponents.

Over the centuries, as war modernized, gas warfare continued to fascinate generals. Medieval warriors used substances like sulfur to distract and disgust their enemies, but a lack of technological prowess made it impossible to manufacture or amass a consistent stockpile of effective chemical weapons.

That changed with the dawn of the industrial era. During the 19th century, developments in chemistry began to yield not just new substances, but more efficient ways to produce them in large quantities. But though proposals to use newly produced chemicals like chlorine, sulfur and cyanide in warfare abounded—and the non-American attendees of the Hague Convention were worried enough about its destructive potential to forbid it in 1899 and 1907—industrial chemical warfare didn’t make its debut until the First World War.

Small amounts of tear gas and xylyl bromide had been used by the Germans at the beginning of the war, but by 1915, Germany was desperate to shake the stalemate along the Western Front. At the Second Battle of Ypres, the Germans fired over 150 tons of chlorine gas across four miles of the front, where it sank into Allied trenches, killing French and Algerian troops.

The attack was essentially a test, says Gerard Fitzgerald, a historian of chemical warfare and visiting scholar at George Mason University, but it was so effective that it soon became an entirely new strategic component of the war. “Things quickly became a different kind of stalemate,” says Fitzgerald.

The public was horrified by the results of the use of chemical weapons like mustard gas and phosgene, which produced psychological terror in addition to burned lungs, seared skin and blindness. An estimated 1.2 million people were exposed to poison gas during World War I, and 91,000 of them died.

As the dust cleared and the world vowed to make World War I the last war, leaders attempted to prohibit the use of gas in another war. The 1925 Geneva Conference banned chemical weapons and the world began to turn its back on poison gas as a weapon of war.

This worldwide repudiation of chemical warfare almost withstood another world war. “Use of such weapons has been outlawed by the general opinion of civilized mankind,” said President Franklin D. Roosevelt in a 1943 address in response to a report that the Axis powers were contemplating the use of poison gas. “I state categorically that we shall under no circumstances resort to the use of such weapons unless they are first used by our enemies.”

Despite the rumors—and a stockpile of sarin gas in Nazi Germany—the Axis powers never did make extensive use of poison gas against military targets during World War II. However, the Nazis did use industrial chemicals against innocent civilians: Zyklon B, an industrial pesticide, and other chemicals were used to murder millions of Jews during the Holocaust.

The international community was shocked by the Holocaust and seemingly committed to halting the use of chemical warfare agents. However, innovation and testing continued during the 20th century. Over the years, the U.S. developed and stockpiled nerve agents like ricin and used herbicides like Agent Orange—most notoriously in the Vietnam War—in defiance of the Geneva Protocol.

Though it’s still unclear what weapons the Soviet Union developed during its secretive, decades-long regime, it’s thought that the USSR did the same, and used chemical agents against civilians during the Soviet-Afghan War. According to the Organization for the Prohibition of Chemical Weapons, “The amount of chemical weapons held by [the U.S. and the USSR] was enough to destroy much of the human and animal life on Earth.”

However, most chemical attacks in the late 20th century were used against smaller targets. Starting in 1963, Egypt used mustard bombs and phosgene, a nerve agent, against military targets and civilians during the Yemeni Civil War. In the 1980s, Iraq used tabun, a nerve agent, and other chemical weapons against Iran and Iraqi Kurds during the Iran-Iraq War.

In 1997, most of the world entered into the Chemical Weapons Convention, which bans stockpiling, developing, creating or using chemical weapons. But, says Fitzgerald, treaties don’t do much to prevent the production or use of industrial chemicals like chlorine. “Chlorine is one of the most highly manufactured industrial chemicals in the world,” he says. “You can’t stop people from making chlorine.”

Nor, it seems, can people be stopped from using it, and other industrial chemicals, during war. Beginning in 2013, Syrian president Bashar al-Assad’s regime is thought to have begun using nerve agents like sarin and poison gases like mustard gas and chlorine against military and civilian targets.

It can take months for investigators to confirm the use of a nerve agent or poison gas, and investigations in Syria have been ongoing since the first attack in 2013. “Chemical weapons are really shifty from an evidence standpoint,” says Fitzgerald. Because they dissipate quickly and must be confirmed via autopsy, gases like chlorine provide plausible deniability for the leaders who choose to use them. And despite international outcries against their use, today their most successful use is against civilians who have no idea they’re coming.

“People in World War I had gas masks and things and were somewhat equipped to deal with them,” says Fitzgerald. “Nowadays they’re just dropping these things on children, which is a totally different kind of warfare. It’s radically depraved.”