Germany used three major poison gases during World War I: chlorine, phosgene, and mustard gas. Each was deadlier and harder to defend against than the last, introduced in roughly that order between 1915 and 1917. Germany also deployed tear gas agents earlier in the war, but the large-scale use of chlorine at the Second Battle of Ypres in April 1915 marked the beginning of modern chemical warfare.
Tear Gas: The First Experiments
Before the infamous chlorine attacks, Germany tested chemical agents on a smaller scale. On January 31, 1915, German forces fired artillery shells filled with xylyl bromide and benzyl bromide, both tear gas compounds, against Russian troops near the town of Bolimów on the Eastern Front. This was the first mass chemical attack in the history of warfare. The results were underwhelming. The cold winter temperatures likely prevented the chemicals from vaporizing effectively, and while the attack yielded some limited tactical gains, it failed to produce a strategic breakthrough. The German military took this as a lesson in delivery, not a reason to abandon chemical weapons.
Chlorine Gas at Ypres
The weapon that changed everything was chlorine. On April 22, 1915, German troops released 160 tons of chlorine gas from cylinders dug into the front edge of their trenches near Ypres, Belgium. A light northeast wind carried the yellow-green cloud across Allied lines. French and Algerian troops, completely unprepared, were devastated. Witnesses described soldiers blinded, coughing, their faces turned purple, some coughing up green froth from their lungs. Hundreds died in the trenches before anyone understood what was happening.
The chemist Fritz Haber, who would become the central figure in Germany’s chemical weapons program, had championed chlorine for practical reasons: it was already widely used in the German dye industry and available in large quantities. When chlorine reacts with moisture in the lungs, throat, and eyes, it forms hydrochloric acid and other corrosive compounds that burn tissue on contact. At low concentrations, victims experienced tearing eyes, throat irritation, and excessive salivation. At the levels released at Ypres, the gas caused violent coughing, chest pain, vomiting, and fluid buildup in the lungs that could drown a person from the inside.
Chlorine had a significant tactical weakness, though. Its pungent smell and visible yellow-green color gave it away. Troops quickly learned to identify an incoming attack, and even crude protective measures like urine-soaked cloth held over the nose and mouth offered some minimal defense. Germany needed something less detectable.
Phosgene: Invisible and Delayed
In December 1915, Germany introduced phosgene at Ypres. It was a major escalation. Phosgene was colorless, making it nearly impossible to spot, and its faint smell resembled freshly cut hay. The problem for victims was that by the time they noticed the odor, they had already inhaled a dangerous dose, since harmful concentrations fell well below the threshold of detection.
Phosgene was six times more potent than chlorine and could be inhaled in lethal amounts without triggering the immediate coughing and choking that chlorine caused. This was what made it so insidious. A soldier could breathe it in, feel only mild eye watering, and believe he was fine. Hours later, sometimes up to 48 hours later, fluid would begin filling his lungs. By then, the damage was done. This delayed onset made battlefield diagnosis extremely difficult and caught medical officers off guard repeatedly in the early months of its use.
Haber’s institute also developed a tactic called “variegated shelling,” which combined different agents in sequence. First, irritant compounds based on organic arsenic were fired to penetrate gas mask filters and force soldiers to rip off their masks. Then phosgene shells followed, hitting troops with exposed faces.
Mustard Gas: The “King of Battle Gases”
The last and most feared chemical agent Germany deployed was mustard gas, first used in July 1917, again near Ypres. Unlike chlorine and phosgene, which primarily attacked the lungs, mustard gas was a blistering agent. It burned skin on contact, caused severe eye damage that could lead to temporary or permanent blindness, and destroyed the lining of the upper respiratory tract when inhaled. It was persistent, too, lingering in soil, clothing, and water for days or even weeks, turning the battlefield itself into a hazard long after the initial attack.
Mustard gas did not need to be inhaled to cause harm. Liquid droplets settling on exposed skin produced painful chemical burns that blistered and ulcerated, often becoming infected. Soldiers who survived a mustard gas attack faced long, agonizing recoveries. The agent’s persistence also made it a powerful area-denial weapon: contaminated ground remained dangerous, slowing troop movements and forcing defenders to abandon positions even after shelling stopped.
How Germany Delivered the Gas
The delivery methods evolved significantly over the course of the war. The first chlorine attacks relied on pressurized cylinders positioned in the front trenches. Soldiers opened the valves and let the wind carry the gas toward enemy lines. This method was cheap and could release enormous volumes, but it depended entirely on favorable wind conditions. A shift in wind direction could blow the gas back onto German troops, and the cylinders were heavy, difficult to transport, and vulnerable to enemy shelling.
Artillery shells gradually became the preferred delivery method. Gas-filled shells could be fired at specific targets regardless of wind direction, mixed with conventional explosives to create confusion, and deployed in the layered attacks Haber designed. By the later years of the war, both sides had developed sophisticated chemical shelling tactics, with purpose-built shells for each type of agent.
Fritz Haber’s Role
Fritz Haber was the driving force behind Germany’s chemical weapons program. A brilliant chemist who had already developed the process for synthesizing ammonia from air (work that would later win him a Nobel Prize), Haber saw chemical warfare as the key to breaking the stalemate of trench warfare. After the initial chlorine attack at Ypres succeeded beyond expectations, he transformed his Kaiser Wilhelm Institute for Physical Chemistry in Berlin into a full-scale chemical weapons research center.
His involvement went far beyond the laboratory. Haber personally supervised gas production facilities, served as chemical advisor to the Ministry of War, and was embedded in the staff of the gas units’ military commander. By November 1915, he headed the “Central Office for Chemical Concerns” in the Artillery Division. His institute developed phosgene and mustard gas as follow-on weapons and designed the combination shelling tactics that made gas masks less effective. His role was controversial even at the time, widely seen as a violation of international law, and his wife, the chemist Clara Immerwahr, died by suicide shortly after the Ypres attack in what many historians connect to her opposition to his work.
The Human Cost
Chemical weapons caused an estimated 1.3 million casualties during World War I across all combatant nations, with roughly 90,000 deaths. The majority of gas casualties survived but suffered lasting health consequences: chronic bronchitis, damaged lungs, impaired vision, and skin scarring. Phosgene alone is estimated to have been responsible for around 80% of chemical fatalities during the war, despite chlorine getting more historical attention due to its dramatic first use. Many survivors experienced recurring respiratory problems for the rest of their lives, and the psychological trauma of gas attacks, the helpless terror of an invisible, suffocating threat, left deep marks on an entire generation of soldiers.