The enemy wasn’t Syrian rebels. It was the weevil, a voracious beetle found in fields and orchards.

Ask a gardener about weevils. Or Orkin, the pest control experts, which says “they can be very destructive, and their damage is often very expensive.”

In the mid-1930s, they were a problem on German farms. The government, forced to buy expensive pesticides from overseas, turned to a scientist at Bayer — yes, that Bayer, the aspirin one — to develop a cheaper alternative.

His name was Gerhard Schrader.

In 1936, he was 33, with a “pale moon face, shrewd eyes framed by over-sized glasses, dark hair slicked over a broad forehead, prominent cheeks, and a wide, amiable mouth,” according to Jonathan Tucker’s book, “War of Nerves.”

As head of Bayer’s plant protection group, Schrader got to work mixing various molecules, trying to concoct a substance strong enough to kill pests but spare animals and humans. He tinkered for months with compounds before he had a eureka moment — add cyanide to the mix.

He quickly developed a splitting headache and struggled to breathe. Suddenly, he could barely see. “He examined his eyes in a mirror and discovered that his pupils had constricted to pinpoints,” Tucker wrote, “giving him an eerie, zombie-like appearance.”

Schrader spent three weeks recovering. He had failed. But by accident, he also succeeded.

IG Farben, the drug conglomerate that owned Bayer, reported Schrader’s discovery to the German military. When German army scientists analyzed Schrader’s concoction, they were inspired by its toxicity to name it “tabun, after the German word for taboo, Tabu,” according to the trade journal Chemical & Engineering News.

Mustard gas took a long time to kill — hours or even days. Tabun needed about 20 minutes. The Third Reich was so impressed with Schrader’s work that he was granted a bonus worth about $20,000, the trade journal wrote.

As the army worked to weaponize tabun, Schrader went back to work in his lab trying to find more pesticides.

This time, he came up with a compound that was 10 times as toxic. Schrader called it sarin.

The German military authorized construction of a sarin factory in 1943. High-level officers wanted to use it during the war. “There is no doubt that a city like London would be plunged into a state of unbearable turmoil that would bring enormous pressure to bear on the enemy government,” one colonel said at the time, according to the chemical journal.

Hitler declined. There’s never been a totally clear explanation for why, but some historians theorize it was because he had witnessed the horrors of mustard gas while fighting in World War I.

In time, other countries would develop sarin, including the United States. In 1993, more than 160 countries signed the Chemical Weapons Convention, which banned production and stockpiling of chemical weapons. But bad actors have used it. Saddam Hussein, for one. And of course Aum Shinrikyo, a cult that used sarin in a 1995 attack on the Tokyo subway, killing 12 people and sickening more than 5,000.

This week, it was unleashed by Syrian President Bashar al-Assad in in a chemical weapons attack that left scores of men, women and children dead, triggered U.S. missile strikes, and raised tensions with Russia.

Schrader died in 1990. But his invention is still killing.

Read more on Retropolis: