At the dawn of the 20th century, the world’s military powers worried that future wars would be decided by chemistry as much as artillery, so they signed a pact at the Hague Convention of 1899 to ban the use of poison-laden projectiles "the sole object of which is the diffusion of asphyxiating or deleterious gases."

Yet from the very start of World War I, both the Allies and Central Powers deployed noxious gasses to incapacitate the enemy or at least strike fear into their hearts. After early failed efforts by the French and German armies to use tear gas and other irritants in battle, the first successful gas attack was launched by the Germans against the British at the Second Battle of Ypres on April 22, 1915.

As the battle began, the Germans released 170 metric tons of chlorine gas from more than 5,700 cylinders buried in a four-mile line across the front. British officer Martin Greener described the horror of that first large-scale gas attack to the Imperial War Museum.

“[T]he next thing we heard was this sizzling—you know, I mean you could hear this damn stuff coming on—and then saw this awful cloud coming over. A great yellow, greenish-yellow, cloud. It wasn’t very high; about I would say it wasn’t more than 20 feet up. Nobody knew what to think. But immediately it got there we knew what to think, I mean we knew what it was. Well then of course you immediately began to choke, then word came: whatever you do don’t go down. You see if you got to the bottom of the trench you got the full blast of it because it was heavy stuff, it went down.”

None of the British soldiers at Ypres had gas masks, resulting in 7,000 injuries and more than 1,100 deaths from chlorine gas asphyxiation. Many of the deaths occurred when panicked victims rushed to drink water for relief from the burning gas, which only made the chemical reaction worse, flooding their throats and lungs with hydrochloric acid.

The British reaction to the German gas attack was “outrage,” says Marion Dorsey, a history professor at the University of New Hampshire and author of A Strange and Formidable Weapon: British Responses to WWI Poison Gas. “Did [the Germans] technically violate the Hague Convention,” which only specifically banned projectiles filled with poison gas? “No. But did they violate the spirit of the ban? Absolutely.”

Sir John French, commander in chief of the British Expeditionary Force, decried the attack as evidence of German barbarity: “All the scientific resources of Germany have apparently been brought into play to produce a gas of so virulent and poisonous a nature that any human being brought into contact with it is first paralyzed and then meets with a lingering and agonizing death.”

Before British troops received proper gas masks with rubber seals called box respirators, they were equipped with stop-gap solutions, like thick gauze pads that were strapped tightly over the mouth. A stretcher bearer at Ypres named William Collins described the pads as more suffocating than the gas:

“I found that in using it in the gas cloud that after a couple of minutes one couldn’t breathe and so it was pushed up over the forehead and we swallowed the gas. And could only put the thing back again for very short periods. It was not a practical proposition at all.”

It wasn’t long before British military officers like the French changed their stance on chemical warfare. If the Germans were going to sink as low as to use gas, then why should the Allies take the high ground? Soon after French made his public statement about the barbarity of German gas attacks, he wrote a private cable to Lord Kitchener, the British Secretary of State for War: “We are taking every precaution we can think of but the most effective would be to turn their own weapon against them & stick at nothing.”

Kitchener wasted no time in developing Britain’s own chemical arsenal. He founded Porton Down, a research facility in the English countryside dedicated to defending Allied troops against gas attacks and stockpiling their own gas weaponry for use against the Germans.

“The British policy was to respond in kind to German gas attacks but never to escalate the war,” says Dorsey.

In late September 1915, the British tried to give the Germans a dose of their own medicine at the Battle of Loos, with little success. The Royal Engineers released chlorine gas an hour before the infantry was scheduled to attack, but the winds shifted, sending clouds of chlorine back toward the British line and forming a toxic fog in no man’s land.

“The gas hung in a thick pall over everything, and it was impossible to see more than ten yards,” wrote one British officer at Loos. “In vain I looked for my landmarks in the German line, to guide me to the right spot, but I could not see through the gas.”

While chlorine gas could kill in concentrated amounts, it was more or less neutralized with the widespread deployment of gas masks by 1917. By that point, however, both sides had discovered far more fatal and crueler chemicals: phosgene and mustard gas.

Phosgene is an irritant that’s six times more deadly than chlorine. Instead of announcing its presence in a yellow-green cloud, phosgene is colorless and takes its time to kill. Victims don’t know they’ve been exposed until days after inhaling it, at which point their lungs fill with fluid and they suffocate. The Germans were the first to use phosgene in battle, but the Allies made it their primary chemical weapon later in the war.

Mustard gas was an entirely new kind of killer chemical. It’s not an irritant, but a “vesicant,” a chemical that blisters and burns the skin on contact. Even if soldiers wore gas masks to protect their lungs, mustard gas would seep into their woolen uniforms and even burn through the soles of their boots, says Dorsey.

By June 1918, the Allies were employing mustard gas as a last-ditch effort to break the stalemate at Ypres. A young Adolf Hitler was among the German troops injured and temporarily blinded by those attacks.

By war’s end, an estimated 6,000 British troops had been killed by gas, a fraction of the 90,000 total World War I deaths from chemical weapons, more than half of which were suffered by the Russians, who had limited access to gas masks.

In the immediate aftermath of World War I, as nations mourned the deaths of tens of millions of soldiers and civilians, most military leaders accepted that chemical weapons would continue to be part of the new barbarity of warfare. But that sentiment was countered by a growing antiwar movement that pushed for arms control treaties and greater diplomacy.

In 1925, the League of Nations adopted the Geneva Protocol, which prohibited the use of chemical and biological agents in war but did not stop nations from continuing to develop and stockpile such weapons.

During World War II, Nazi Germany killed millions of Concentration Camp victims in gas chambers pumped full of carbon monoxide or the pesticide Zyklon B, but decided against deploying a new class of nerve gases in battle for fear of Allied retaliation. China has also accused Imperial Japan of firing artillery packed with mustard gas and other blistering agents during World War II. In the Vietnam War, the United States used the chemical weapons napalm and Agent Orange to terrible effect.

The current ban on chemical weapons was signed into international law by two conventions in 1972 and 1993.