Hitler certainly had the opportunity to use sarin in World War II. The Nazis were actually the ones to develop the deadly nerve agent—accidentally. In late 1938, the German scientist Gerhard Schrader was tasked with inventing a cheaper pesticide to kill the weevils that were damaging German fields and orchards. By mixing phosphorus with cyanide, he came up with a substance that was way too toxic to use for agriculture purposes.

After Schrader’s employer, drug conglomerate I.G. Farben, informed the German army of his discovery, some impressed army scientists dubbed the liquid “tabun,” after the German word for taboo. Back in the lab, Schrader tinkered some more and came up with something even more toxic. He called the new substance sarin, an acronym for the names of the four scientists who developed it.

By the end of World War II, Nazi Germany had produced some 12,000 tons of the deadly chemical compound, enough to kill millions of people. From early in the conflict, high-level military officers pressed Hitler to use sarin against their adversaries. But despite such pressure, Hitler declined to employ it as a chemical weapon against the Allied Powers.

As reported in the Washington Post, some historians have traced this reluctance to Hitler’s own experience as a soldier during World War I. Though Germany was the first to unleash chlorine gas on French troops during the Second Battle of Ypres in April 1915, Britain and France would also employ chlorine and mustard gas during the Great War, generating widespread outrage over the new horrors of chemical warfare.

In his biography of the Nazi leader, the historian Ian Kershaw described how Hitler himself fell victim to a mustard gas attack near Ypres on the night of October 13-14, 1918: “He and several comrades, retreating from their dug-out during a gas attack, were partially blinded by the gas and found their way to safety only by clinging to on to each other and following a comrade who was slightly less badly afflicted.” After the attack, Hitler was transported from Flanders to a military hospital in Pomerania, where he would learn the devastating news of Germany’s surrender.

The idea that Hitler would have objected to using poison gas on the battlefield on ethical grounds may seem blatantly inconsistent with the fact that Nazis were systematically using Zyklon B and other chemical agents to exterminate millions of people in the gas chambers. But even setting this aside, there’s little to no solid historical evidence linking Hitler’s wartime experience to his reluctance to use sarin against the Allies 20 years later.

Other factors may have been involved. Germany’s Blitzkrieg military strategy, which had so far been successful, involved sudden attacks by tanks and bombers followed swiftly by invading foot soldiers. If those bombers used sarin or another chemical weapon, they would have contaminated the same area their troops would then have had to march into.

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More importantly, perhaps, Hitler must have known that if he used chemical weapons, his adversaries would retaliate in kind. British Prime Minister Winston Churchill, for one, had long argued in favor of the use of such weapons to shorten military conflicts. “I cannot understand this squeamishness about the use of gas,” he wrote in a memo in 1919, when he was Britain’s secretary of war. “It is not necessary to use only the most deadly gasses: gasses can be used which cause great inconvenience and would spread a lively terror and yet would leave no serious permanent effects on most of those affected.”

Historian Richard Langworth has emphasized that Churchill believed using (non-lethal) chemical weapons could actually be a more humane way of doing battle. In another memo written around the same time, Churchill argued: “Gas is a more merciful weapon than high explosive shell, and compels an enemy to accept a decision with less loss of life than any other agency of war.”

During World War II, Churchill was always prepared to use chemical weapons, but only if the enemy unleashed them first. In February 1943, when London learned the Germans might use gas against the Russians in the Donets Basin, Churchill wrote to his Chiefs of Staffs Committee: “In the event of the Germans using gas on the Russians…We shall retaliate by drenching the German cities with gas on the largest possible scale.”

But for whatever reason, Hitler chose not to take that step—even as Nazi factories secretly stockpiled munitions packed with the deadly nerve agent, and even as the tide of the war turned increasingly against Germany.