Adolf Hitler gassed and killed six million Jews during World War II -- a genocide that makes his reluctance to use sarin against his military adversaries an enduring mystery.

It wasn't because he was somehow less evil than Syrian president Bashar Assad, despite White House Press Secretary Sean Spicer's tone-deaf statement Tuesday that "someone as despicable as Hitler" didn't use chemical weapons the way Assad did.

And it wasn't because Hitler didn't have sarin. A German scientist had stumbled onto sarin while experimenting with compounds in an attempt to kill beetles. The German military built a sarin factory in 1943. Officers pleaded with Hitler to use it.

He didn't.

Why?

Over the years, historians (armchair and scholarly) and psychologists have speculated that that maybe Hitler didn't use sarin because he was a victim of a mustard gas attack in 1918, during World War I, and knew the misery of such weapons.

"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 on to each other and following a comrade who was slightly less badly afflicted," Ian Kershaw wrote in his critically acclaimed Hitler biography.

Hitler described the blindness in his 1924 autobiography, Mein Kampf.

"Towards morning," Hitler wrote, "I also began to feel pain. It increased with every quarter of an hour, and about seven o'clock my eyes were scorching . . . A few hours later my eyes were like glowing coals, and all was darkness around me."

Hitler was taken to a hospital, where he eventually recovered and learned that Germany had been defeated. The world knows what happened next.

But there's not much, if any, historical evidence to suggest that Hitler vetoed sarin because of his mustard gas experience.

But there is a wealth of historical record about Hitler's archenemy in World War II: Winston Churchill.

Churchill embraced the use of chemical weapons during World War I.

"I cannot understand this squeamishness about the use of gas," the future British prime minister wrote in a 1919 memo. "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."

If Hitler used sarin on the battlefield, Churchill likely would have retaliated with his own chemical weapons.

War is chess. Hitler would have sacrificed a lot of pieces that he couldn't afford to lose.

He never made that move.