Congress directed their eradication in 1986, and the Chemical Weapons Convention of 1997, which the U.S. joined with 192 countries to forbid creation and stockpiling of such weapons, required elimination of the weapons by Sept. 30.

The United States was the last participating country to have finished culling its chemical weapons stocks.

Nearly 100,000 people were killed by chemical weapons in World War I, according to the United Nations. Since then, the weapons have been responsible for mil casualties worldwide, the U.N. says.

The process has been long and complex, officials said. The munitions mix explosives and chemical agents, and each can become unstable over time. Legislation required the Defense Department to determine ways to destroy the weapons and agents without incineration, the Pentagon said.

“These weapons were not designed to be taken apart,” Kingston Reif, the deputy assistant secretary of defense for threat reduction and arms control, told reporters Monday. “They had to be painstakingly disassembled in reverse.”

The process, described in part as “biotreatment,” was carried out the U.S. Army’s Pueblo Chemical Depot in Colorado and at the Blue Grass Army Depot in Kentucky.

The convention to ban chemical weapons has not ensured they have disappeared from military inventories. Syrian troops, for instance, launched a chlorine gas attack in 2018 hat killed 43 civilians, a chemical weapons watchdog concluded in January, one of hundreds of chemical weapons attacks linked to the government of Bashar al-Assad.