President Joe Biden announced on Friday that the United States had destroyed the last of its declared chemical weapons stockpile, bringing an end to a decades-long effort to remove the deadly weapons first employed on a massive scale in World War One.

The United States and other signatories are required by the Chemical Weapons Convention, which was adopted by the United States Senate in 1997, to destroy their chemical weapons stockpiles by September 30, 2023.

“Today, I am proud to announce that the United States has safely destroyed the final munition in that stockpile—bringing us one step closer to a world free from the horrors of chemical weapons,” Biden said in a written statement released by the White House.

The U.S. has been destroying its remaining stockpiles at U.S. Army Pueblo Chemical Depot in Pueblo, Colorado, and Blue Grass Army Depot (BGAD) in Richmond, Kentucky.

In 2022, the last M55 rocket with the VX nerve agent was destroyed at the plant in Kentucky.

The U.S.’ stockpile of chemical warfare agents reached nearly 40,000 tons by 1968, according to the Centers for Disease Control and Prevention.

“Chemical weapons are responsible for some of the most horrific episodes of human loss. 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,” U.S. Senate Republican leader Mitch McConnell said in a statement.

Chemical weapons came to the fore during World War One, which became known as the “chemist’s war.”

According to the United Nations, chemical weapons killed nearly 100,000 people during World War One and have caused more than 1 million casualties around the world since then.