The United States’s Chemical Warfare Service readied hundreds of thousands of mortar shells and artillery rounds filled with mustard gas in the 1940s. During the Cold War, even more lethal chemical weapons followed: artillery and rockets filled with VX and GB, better known as Sarin, nerve agents that, with as little as a few drops, can be deadly.

These munitions would make up the United States’s chemical weapons arsenal, one of the biggest in the world.

It’s all gone now. This summer, on July 7, at the Blue Grass Chemical Agent-Destruction Pilot Plant in Kentucky, the last M55 rocket, filled with GB, was dismantled. With it went the entirety of the US’s declared chemical munitions stockpile.

The United States achieved this just shy of its September 30 deadline under the Chemical Weapons Convention (CWC), the 1997 international treaty that bans the production, use, and stockpiling of these weapons. The US was the last country party to the treaty to eliminate its declared chemical weapons stockpile, destroying the kinds of agents and munitions once hoarded for use on the battlefield.

The world still has chemical weapons — in countries that never signed the treaty, scattered in old war zones, and likely in nations that have broken their treaty promises.

But the US certification is still a huge achievement for America, and for the world.

The US had some 30,000 tons of chemical warfare agents at the time of the CWC ratification. The US learned quickly that agreeing to eliminate chemical weapons was one thing. Actually doing so was far more complex. “These are weapons that were built to be used, not destroyed,” said Sonia Ben Ouagrham-Gormley, an expert in weapons programs and an associate professor at George Mason University.

That treaty effort stretched more than 25 years, though the US had grappled with how to dismantle its arsenal safely and effectively even before that. The US wasn’t alone in needing extensions under the CWC, but the American experience was uniquely lengthy and complicated.

Local, state, and federal lawmakers all got involved, as did environmental and community activists who questioned and challenged how the US Army planned to destroy toxic agents in the places where they and their families lived. It was akin to a “not in my backyard” movement with something close to existential stakes. These organizers used their protests to create new policies and influence the technology and methods used to destroy these munitions. Early opponents became community watchdogs for a global agreement so that the treaty’s mission — the safe elimination of an entire class of weapons — reflected the desires of the public it was intended to protect.

These debates and delays weren’t exactly predicted when countries signed on to the Chemical Weapons Convention, but they helped reveal one of the biggest challenges of disarmament: The decision to produce weapons of mass destruction is not easily unraveled or undone. Chemical munitions were designed to kill, not to be disassembled and decontaminated. It took decades to eliminate America’s chemical weapons arsenal because, as dangerous as these weapons are to make and to store, they are all that much harder to destroy.

Craig Williams remembers the US Army hosting members of the local community for a meeting in February 1984 on the grounds of the Blue Grass Army Depot in Richmond, Kentucky. About 300 people showed up. “The Army got up,” Williams, the co-chair of the Kentucky Citizens’ Advisory Commission, recalled, “and they explained that there were chemical weapons stored on the facility’s grounds, and they planned to dispose of them by incinerating them. And did anybody have any questions?”

Many people had many, many questions, Williams said. For good reason. Blue Grass was one of nine chemical weapons depots maintained by the United States (there were eight within the continental US and one on Johnston Atoll in the Pacific). Communities like Williams’s knew of these military facilities, but what was being stored in those lumps on the landscape wasn’t widely advertised. Many found out about the chemical weapons close to their neighborhoods when the Army said it wanted to destroy them.

Williams had just collided with the start of the latest, maybe most contentious, chapter of the US’s efforts to maintain its chemical weapons stockpile, one that began nearly a decade before the CWC even opened up for signatures.

The United States used chemical weapons in World War I, though they were foreign-made munitions from its allies. That use of poisonous gas on European battlefields helped prompt countries to create the Geneva Protocol of 1925, which banned poisonous gasses and biological agents in war. The US did not sign on at the time and continued researching and developing chemical weapons, although it wasn’t a huge priority for the military until World War II. Washington did not deploy chemical munitions in World War II, though it “had supplies of agents and equipment with which they could have waged warfare energetically if necessary,” according to The Chemical Warfare Service: From Laboratory to Field.

Most of those World War II-era weapons were blister agents, like mustard, which can cause burns or blisters, damaging the eyes or lungs; they were intended to slow enemy troop movements. During the Cold War, the US began experimenting with nerve agents in rockets and artillery, things like GB that, when released, acted fast and were almost assuredly lethal.

Both the US and the then-Soviet Union ultimately built huge chemical stockpiles, each with, at points, an estimated 30,000 to 40,000 tons of chemical agents.

By the early 1960s, though, these weapons started to fall out of favor in the US. America still felt it necessary to have chemical weapons in case the USSR used them, but the Cold War emphasis was on America’s nuclear arsenal. There were also some public mishaps — like an alleged open-air VX test in Utah that killed or injured thousands of sheep — and public anger over the use of herbicides like Agent Orange during the Vietnam War, which created lasting harm and health issues for both US veterans and civilians in Vietnam, Laos, and Cambodia.

These forces helped push Congress to pressure the Nixon administration to review the entirety of the US biological and chemical weapons programs. In 1969, Nixon renounced biological weapons — eventually leading to an international treaty banning those — and the US reiterated a no-first-use policy for lethal and incapacitating chemicals (meaning, Washington would only use them if Moscow did first) and halted the production of new chemical weapons.

Yet it wasn’t as simple as hitting pause. All weapons have a shelf life, and chemical munitions are no exception. They age, they degrade, they can leak. You can’t just put them in storage and forget about them. Maintaining an adequate arsenal also requires disposing of its faulty components.

The solution was mostly the sea. In the late 1960s, the US undertook Operation CHASE (“Cut Holes and Sink ’Em”). It is what it sounds like: Load a bunch of chemical weapons or ammunition on an old ship and sink it all. The other options, though, were worse: burning chemical weapons in the open air or burying them on land.

These operations also started to come under scrutiny amid a growing environmental movement. In the 1970s, Congress more tightly regulated the disposal of chemical weapons, forcing health and safety reviews, and eventually outlawing the sea dumps. This solved one problem but not the other: a bunch of old, crumbling chemical weapons, sitting in storage.

Which was the Army’s dilemma when it showed up near Williams’s hometown. By that point, in the 1980s, the Pentagon said the US stockpile was barely usable. The munitions didn’t work with the current-day launchers. It was all a bunch of crap, albeit very, very dangerous crap that needed to be closely monitored.

The military’s plan was to replace the old stocks with a “binary” chemical munition. It sold these newer weapons as a more stable, “safer” version because instead of filling up an artillery shell with a lethal toxin, these munitions separated the chemical compounds so that they became a deadly nerve agent only after being fired, making them easier to transport, store, and, if necessary, get rid of.

Congress was less convinced. The US had stopped producing new chemical weapons and now indicated it wanted a worldwide ban. The Pentagon proposed upgrading an arsenal the US had by now promised it would never use.

Lawmakers found a kind of compromise: For every new binary weapon the military wanted, it would have to get rid of one old munition first.

The Army had already begun piloting methods of destroying chemical weapons at this point. One was incineration, which uses very, very high temperatures to destroy the chemical agent (and also treat the munition). The Army began employing on a small scale starting in the 1970s.

Now the Army planned to scale up incineration. And when the military told people who lived near these chemical depots what they proposed to do, a lot of people in those communities thought some version of: You’re going to do what with what? Where?

Williams felt the Army didn’t have any satisfactory answers when he and others pressed it on the mechanics of incineration. “Simple things like, you know: What comes out of the stack? How does the technology work?” Williams recalled. “And they were like, well, just, you know, ‘Trust us.’”

This sense of distrust and skepticism existed elsewhere, too, in addition to the fear that the Army wasn’t listening to their concerns about possible pollution or health effects.

Rufus Kinney, an activist in Alabama, joined protests, including a ribbon-burning with civil rights leaders at the chemical depot site in Anniston, Alabama. As Kinney noted, the depot was near a predominantly Black neighborhood that had been poisoned for decades by Monsanto; why would this time be different? In Pueblo, Colorado, home to another depot, Irene Kornelly, chair of the Colorado Citizens’ Advisory Commission, recalled how farmers and ranchers worried about the possibility of tainted food supplies.

And it made some sense: Incineration called to mind industrial processes with smelly stacks puffing out dark smoke. The process to destroy chemical weapons was not the same as “take trash from the local community and throw it in and burn it up,” said Michael Greenberg, a professor emeritus at Rutgers and a member of the National Research Council Committees that consulted on the destruction of the US chemical weapons stockpile.

The incinerators expose toxic agents to very, very, very high temperatures, and through a series of steps, the end product becomes harmless. Incineration was the Army’s preferred method of disposal. They argued it could be tightly controlled and regulated and prevented the possibility of any chemical agent re-forming. The process included safeguards to protect workers and communities, such as stringent monitoring protocols and airflow systems that prevented chemicals from being released.

But many activists said they didn’t feel as though their concerns were adequately addressed: What if something went wrong in the process? The military may be monitoring what’s being released, but how confident should affected communities be that everything was being detected?

The Army essentially told people, “‘We’re the technical experts so you need to follow our direction,’” said Robert Futrell, professor of sociology at the University of Nevada-Las Vegas, who has researched the destruction of chemical weapons and grew up near the Blue Grass depot. “But there’s a question that I think the citizens were raising as well: ‘You might be the technical experts, but are you asking all the right questions?’”

As this was unfolding at home, the United States was getting out of the chemical weapons game altogether at the international level. The US and the USSR negotiated an arms control agreement on chemical weapons, signed in 1990, in which they agreed to make no new weapons and drastically reduce their stockpiles by 2002.

This brought momentum to a global treaty. The CWC opened for signatures in 1993. It prohibited the production, development, and use of chemical weapons, and notably included a robust verification and inspection regime. The US and Russia both signed. More than 190 states are now party to the treaty.

The CWC went into force in 1997. It was a huge global accomplishment, the outlawing of an entire class of weapons, one considered uniquely dangerous and horrific. Unlike the Biological Weapons Convention before it, countries agreed to robust verification metrics, such as on-site inspections, including of industry, to prevent any materials from being repurposed for weapons use.

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