(PUEBLO, Colo.) — The stockpile of obsolete chemical weapons stored at the Pueblo Chemical Depot since the 1950s has been eliminated.

Walton Levi, the site project manager said the final round was destroyed on Thursday, June 22 ending a process that began in March 2015.

With the stockpile destroyed, the closure phase begins, which will take three to four years according to the Depot. the closure timeline is for the disposal of secondary wastes, decontamination and decommissioning of facilities and equipment, the transfer of property, and the demolition of certain facilities.

“With this milestone, the United States reinforces its commitment to achieving a world free of chemical weapons. We have a national security imperative as well as a moral responsibility to reduce and eventually eliminate the threat posed by these weapons of mass destruction. As the Department continues working toward this shared goal, this accomplishment shows the United States is leading by example and is committed to the highest standards of transparency and public safety,” said Dr. William LaPlante, Under Secretary of Defense for Acquisition and Sustainment.