RICHMOND, Ky. — The last of the United States’ declared chemical weapons stockpile was destroyed at a sprawling military installation in eastern Kentucky, Senate Republican leader Mitch McConnell announced Friday, a milestone that closes a chapter of warfare dating back to World War I.

Workers at the Blue Grass Army Depot in Kentucky destroyed rockets filled with GB nerve agent, completing a decadeslong campaign to eliminate a stockpile that by the end of the Cold War totaled more than 30,000 tons.

“Chemical weapons are responsible for some of the most horrific episodes of human loss,” McConnell said in a statement. “Though the use of these deadly agents will always be a stain on history, today our nation has finally fulfilled our promise to rid our arsenal of this evil.”

The weapons’ destruction is a major watershed for Richmond, Kentucky and Pueblo, Colorado, where an Army depot destroyed the last of its chemical agents last month. It’s also a defining moment for arms control efforts worldwide.

The U.S. faced a Sept. 30 deadline to eliminate its remaining chemical weapons under the international Chemical Weapons Convention, which took effect in 1997 and was joined by 193 countries. The munitions destroyed in Kentucky were the last of 51,000 M55 rockets with GB nerve agent — a deadly toxin also known as sarin — stored at the depot since the 1940s.

By destroying the munitions, the U.S. is officially underscoring that these types of weapons are no longer acceptable on the battlefield and sending a message to the handful of countries that haven’t joined the agreement, military experts say.

Friday’s announcement came as the Biden administration decided to provide cluster munitions to Ukraine, a weapon that two-thirds of NATO countries have banned because it can cause many civilian casualties. National security adviser Jake Sullivan said Ukraine has promised to carefully use the munitions — bombs that open in the air and release scores of bomblets.

“One thing that we’re really proud of is how we’re finishing the mission. We’re finishing it for good for the United States of America,” said Kim Jackson, manager of the Pueblo Chemical Agent-Destruction Pilot Plant.

Chemical weapons were first used in modern warfare in World War I, where they were estimated to have killed at least 100,000. Despite their use being subsequently banned by the Geneva Convention, countries continued to stockpile the weapons until the treaty calling for their destruction.

In southern Colorado, workers at the Army Pueblo Chemical Depot started destroying the weapons in 2016, and on June 22 completed their mission of neutralizing an entire cache of about 2,600 tons of mustard blister agent.

The projectiles and mortars comprised about 8.5% of the country’s original chemical weapons stockpile of 30,610 tons of agent.

Nearly 800,000 chemical munitions containing mustard agent were stored since the 1950s inside row after row of heavily guarded concrete and earthen bunkers that pock the landscape near a large swath of farmland east of Pueblo.

The weapons’ destruction alleviated a concern that civic leaders in Colorado and Kentucky admit was always on their minds.

“Those (weapons) sitting out there were not a threat,” Pueblo Mayor Nick Gradisar said. But, he added, “you always wondered what might happen with them.”

In the 1980s, the community around Kentucky’s Blue Grass Army Depot rose up in opposition to the Army’s initial plan to incinerate the plant’s 520 tons of chemical weapons, leading to a decadeslong battle over how to dispose of them.

They were able to halt the planned incineration plant, and then, with help from lawmakers, prompted the Army to submit alternative methods to burning the weapons.

Craig Williams, who became the leading voice of the community opposition and later a partner with political leadership and the military, said residents were concerned about potential toxic pollution from burning the deadly chemical agents.

Williams noted that the military eliminated most of its existing stockpile by burning weapons at other, more remote sites such as Johnston Atoll in the Pacific Ocean or at a chemical depot in the Utah desert.

But the Kentucky site was adjacent to Richmond and only a few dozen miles away from Lexington, the state’s second-largest city.

The Kentucky storage facility had housed mustard agent and the VX and sarin nerve agents, much of it inside rockets and other projectiles, since the 1940s.

The state’s disposal plant was completed in 2015 and began destroying weapons in 2019. It used a process called neutralization to dilute the deadly agents so they can be disposed of.

Workers at the Pueblo site used heavy machinery to meticulously — and slowly — load aging weapons onto conveyor systems that fed into secure rooms where remote-controlled robots did the dangerous work of eliminating the toxic mustard agent, which was designed to blister skin and cause inflammation of the eyes, nose, throat and lungs.

Robotic gear removed the weapons’ fuses and bursters before the mustard agent was neutralized.