In reality, all weapons of war are weapons of mass destruction, from guns and bombs to bayonets and arrows. Whenever one is fired, the idea is to inflict lethal harm on the person on the other end.

War is war. And the intense hate that marks it means nations may use anything and everything to try and defeat the enemy. Could this happen today? Could some of history’s worst weapons again be used, and Vladimir Putin unleash chemical weapons on Ukraine?

We don’t yet know. What we do know is that warfare took a hideous turn when the Germans introduced waves of chlorine gas over the battlefield at Ypres in Belgium in April, 1915. From then on, most of the nations developed and used chemical weapons in World War I. Mustard gas and phosgene were used by the Germans and the British. Lewisite (nicknamed the ‘dew of death’) was discovered by U.S. scientists, and manufactured near Willoughby, Ohio. Lewisite was never used in battle, but we manufactured tons of it. As 1918 was winding down it was shipped east by rail up over the mountains to Baltimore, where it was loaded on vessels headed for France. The war ended before the ships got there, and tons of Lewisite was dumped in the ocean.

Awful as these weapons had been, chemists continued to search for toxic substances. By the time the United States was drawn into World War II, a chemical corps was working around the country. A group at UCLA studied ways to deliver such weapons because it was thought they might be needed to conquer Japan.

Friends of mine later served in that corps, one at the army’s Dugway Proving Ground in Utah, then in Florida and finally Panama. His job was to quickly go to an area where a toxic compound had been dropped from the air, and measure its concentration with an infrared spectrometer at the height of an average Japanese soldier. Accidents were frequent, and he was burned in Panama.

Ironically, the most toxic chemicals then known weren’t used in World War II: The nerve agents discovered by Gerhard Schrader working for Bayer near the industrial Ruhr in 1936. These included sarin and tabun, compounds so toxic that microscopic quantities would kill humans. Regrettably, they have been used occasionally: Once by a Japanese terrorist in an attack on a Tokyo subway and last year on Alexei Navalny when Putin agents put a novichok, Russian for ‘ new agent’, in his underwear.

Fortunately Navalny was treated in Siberia and then in Berlin, where experts identified the specific agent. Eventually he regained his health and was discharged, only to be locked up again when he returned to Russia. Russian scientists who developed novichok agents claim them to be the deadliest ever, with some variants possibly five to ten times more potent than soman.

Supposedly, producing these weapons, let alone using them, is strictly illegal under international law. Chemical weapons treaties were signed by many nations including Russia, after the fall of the Soviet Union. Chemical stocks were to be destroyed and no new compounds found or made. Yet they have been.

Do we expect the Russians to use chemical weapons in the Ukraine? I’d guess they would. They have a driven and brutal leader, and an invading army that has become bogged down.

Our stash of sarin, soman, and other nerve agents took years to destroy. After World War II ended in 1945, the victorious nations tried to make sure they were at par with every other nation. Sarin and soman were difficult to make, so industrialist W. Peter Grace brought several German chemical experts to the United States where they supervised the synthesis of tons of nerve agents. A few years ago tank cars filled with it were shipped from Colorado to Alabama, where the contents were purified, and then sent back to Colorado for storage.

The bottom line: The world has a significant supply of very toxic agents mostly sitting in storage depots. The only question is who will pull that trigger first. They are hideous weapons. When the Germans first used chlorine at Ypres in 1915, they failed to account for the winds, and the escaping gas blew back on their troops. Nearly a thousand soldiers on both sides were killed by gas in those first attacks. Many more deaths lay ahead.

The German chemical weapons program was directed by Professor Fritz Haber. When Haber’s wife, Clara Immerwahr, found out he was going to accompany German troops east where gas was to be used, she shot herself.

Gas is a terrible, terrible weapon. Let’s hope we don’t see its use again.

Douglas Neckers is an organic chemist, the McMaster distinguished professor emeritus and the founder of the Center for Photochemical Sciences at Bowling Green State University. He is also a former board chair of the Robert H. Jackson Center in Jamestown, N.Y., named for the Supreme Court Justice and the chief U.S. prosecutor during the Nuremberg trials.