The History of Chemical Warfare in WWI

The Weapon Nobody Was Ready For

ON APRIL 22, 1915, soldiers on the Allied side of the Western Front noticed something strange drifting toward them from the German trenches: a yellowish-green cloud, low to the ground, moving with the wind. Within minutes, men were clutching their throats, coughing blood, collapsing in the mud. Those who could run, ran. Those who couldn't died where they lay. It was the first large-scale use of chemical weapons in modern warfare, and it changed the nature of war permanently.

The weapon was chlorine gas. The Germans released 168 tons of it from 5,730 cylinders over a four-mile front near Ypres, Belgium. Within ten minutes, over a thousand men were dead and four thousand more were injured. A two-mile gap opened in the Allied line. The Germans, uncertain of their own success and lacking reserves, didn't exploit it fully. But the damage was done in a different sense: a threshold had been crossed that would prove almost impossible to walk back.

The Science Behind the Horror

Chemical weapons in WWI fell into several categories, each targeting the human body in a different way. Understanding what these substances actually do makes it clear why soldiers feared them more than bullets.

Chlorine attacks the respiratory system. When inhaled, it reacts with moisture in the lungs to form hydrochloric acid. Victims drown in their own fluid. Phosgene, introduced later and responsible for roughly 80-85% of chemical warfare deaths in WWI, was harder to detect. It smelled faintly of cut hay and often produced no immediate symptoms, which meant soldiers kept fighting until their lungs gave out hours or days later.

Then came mustard gas, introduced by Germany in 1917. It was not a gas at all, technically, but an oily liquid that evaporated slowly and settled into the environment. It didn't kill quickly. Instead, it blistered skin, blinded eyes, and scorched airways. Soldiers could lie in contaminated soil for hours before symptoms appeared. Mustard agent persisted in the environment for days, making terrain unusable. Its psychological effect was enormous: even the rumor of it caused panic.

Both Sides Raced to Keep Up

The Allies responded quickly. Within weeks of Ypres, British and French chemists were working on their own agents. By the end of the war, both sides had deployed dozens of different chemical compounds in a continuous arms race that mirrored the trench warfare stalemate itself.

France was the first Allied power to use chemical agents, deploying xylyl bromide (a tear gas) as early as 1914, before the war had even settled into trench patterns. Britain developed its own chlorine attacks and later became a major producer of phosgene. By 1918, roughly a quarter of all artillery shells fired on the Western Front contained chemical agents.

The Germans, who had initiated large-scale chemical warfare, also suffered from it. By the war's end, no side had a meaningful advantage from chemical weapons. What they had created instead was a shared nightmare that every soldier lived with regardless of nationality.

Fritz Haber: The Man Who Made It Possible

No figure in the history of chemical warfare is more complicated than Fritz Haber. A German Jewish chemist, he had already changed the world before 1914 by developing the Haber-Bosch process for synthesizing ammonia, which made modern agriculture possible by producing artificial fertilizer at scale. He is, in a very real sense, responsible for the food that keeps billions of people alive today.

He is also the man most directly responsible for Germany's chemical warfare program. Haber personally oversaw the first chlorine attack at Ypres, standing near the front to observe results. He believed chemical weapons were a more humane way to end the war quickly, reducing overall casualties by breaking the stalemate. His wife, Clara, herself a chemist, argued with him bitterly over the ethics of his work. Days after the Ypres attack, she shot herself with his military pistol. Haber left for the Eastern Front the next morning.

He won the Nobel Prize in Chemistry in 1918, the same year the war ended. The award was controversial. He died in 1934, fleeing Nazi Germany as a Jew. His extended family later died in concentration camps, killed in part by Zyklon B, a pesticide developed partly from his research into poison gases.

What the Trenches Actually Looked Like

For the ordinary soldier, chemical warfare was a constant, low-level terror. Gas masks improved significantly as the war progressed, but they were uncomfortable, reduced visibility, and had to be carried at all times. A soldier who fumbled with his mask under pressure, or whose mask seal was imperfect, could suffer severe injury or death from concentrations that a properly equipped man would survive.

Gas attacks often came at night or in fog, exploiting low visibility. Artillery crews fired gas shells rather than releasing clouds, which meant there was no visible warning. The distinctive "plop" of a gas shell rather than the crack of a high-explosive round became a sound soldiers learned to recognize and fear. Some accounts describe men being too exhausted to react in time, too traumatized by constant shelling to process the specific threat of a gas round landing nearby.

Long-term survivors carried the damage with them. Veterans who survived mustard gas attacks suffered chronic respiratory problems, recurring skin conditions, and elevated rates of cancer. Some were blinded permanently. The psychological toll was also significant, contributing to what was then called "shell shock" and what we would now recognize as severe PTSD.

The Numbers and What They Mean

Chemical weapons caused roughly 1.3 million casualties in WWI, of whom around 90,000 to 100,000 died. Compared to the roughly 17 million total dead from the war, chemical weapons account for a relatively small percentage of fatalities. This fact sometimes gets used to argue they were less deadly than artillery or machine guns.

That comparison misses the point. Chemical weapons were uniquely terrifying, uniquely unpredictable, and uniquely indiscriminate. They could drift with wind changes and affect rear areas, field hospitals, civilian populations. They were designed to incapacitate rather than kill outright, which meant they generated enormous numbers of wounded men who required intensive care. And they forced changes in tactics, logistics, and equipment that affected every aspect of warfare.

The Road to the Geneva Protocol

By 1918, the war's end brought immediate efforts to outlaw what had just happened. The 1925 Geneva Protocol prohibited the use of chemical and biological weapons in war. It was not perfect: it didn't ban production or stockpiling, only first use. Many countries signed it with reservations that amounted to "we won't use it unless the other side does." And several major powers didn't ratify it until decades later (the United States didn't ratify until 1975).

But it established a norm that, fragile as it is, has shaped international behavior ever since. Chemical weapons have been used in conflicts since 1918 (Italy in Ethiopia, Japan in China, Iraq against the Kurds, Syria against its own population) but each use has been treated as a violation, a crossing of a recognized line, rather than normal warfare.

The Chemical Weapons Convention of 1993 went further, banning production and stockpiling. By 2023, 193 countries had joined it. The norm is not absolute, but it exists, and it traces directly back to the trenches of Ypres and the decades of revulsion that followed what happened there.

Why This History Still Matters

Chemical warfare in WWI is not a historical curiosity. The agents developed between 1914 and 1918 informed later weapons programs. The debates about proportionality, about what counts as a weapon too terrible to use, about how international law can constrain the behavior of states under extreme pressure, all of them started in that mud.

When a government today uses nerve agents against dissidents, or when a non-state actor deploys chlorine in a crowded neighborhood, the response that follows, the international condemnation, the invocation of international law, the calls for accountability, all of it rests on a framework built from the horror of WWI. The soldiers who choked in the yellowish-green cloud at Ypres created, through their suffering, a legal and moral architecture that still shapes how the world thinks about the limits of war.

It is a grim kind of legacy. But it is a real one.