While the United States maintained a stockpile of the agent for a long time, it looks like this weekend the U.S. Army is set to begin destroying it.

From the Associated Press:

The largely automated plant at the military's Pueblo Chemical Depot in southern Colorado will begin destroying about 780,000 chemical-filled artillery shells soon after this weekend, said Greg Mohrman, site manager for the plant. He declined to be specific, citing security concerns and possible last-minute delays. "We've practiced a lot," Mohrman told The Associated Press. "Next week it gets real." Robots will dismantle the shells, and the plant will use water and bacteria to neutralize the mustard agent, which can maim or kill by damaging skin, the eyes and airways. At full capacity, the facility can destroy an average of 500 shells a day operating around the clock. It's expected to finish in mid-2020.

After World War I, chemical weapons like mustard agents were rarely used. The Geneva Protocol, debated in the 1920s and adopted towards the end of the decade, prohibited the use of gases in war, though it failed to address the production or stockpiling of chemical weapons. Italy, under the fascist dictator Mussolini, used chemical weapons against Ethiopia, Japan used them against China during World War II, and the United States and Soviet Union maintained stockpiles they did not use during the war. (While the use of gas was debated, a different weapon of mass destruction instead ushered in the end of the war.) 

Only a few countries used chemical weapons at all after World War II: Egypt, against Yemen in the 1960s, and Iraq's dictator Saddam Hussein, in the 1980s. A later treaty, the Chemical Weapons Convention, was signed in the 1990s, and it prohibited not just the use but the production and stockpiling of chemical weapons as well.

For the most part it's worked, but Syria, under the dictatorship of Bashar al-Assad, used chemical weapons in the ongoing civil war. While many of them were turned over and destroyed in 2013, continued chemical attacks since then show the destruction wasn't complete.

The history of chemical weapons is an especially bleak one, in the generally bleak history of war. Destroying the weapons is one way to close that chapter on history, or at least leave it with fewer authors. In 2013, Popular Science spoke with Al Mauroni, who wrote the book on Chemical Demilitarization about what it takes to destroy chemical weapons. The Pueblo plant, not yet operational at the time of the interview, is one of the major parts of safely neutralizing the mustard weapons, and Mauroni explained the science of neutralization in depth.

With luck, America's chemical weapons stockpile will be gone by the end of the 2020s, a full century after the first treaty prohibiting their use in war.