PUEBLO, Colo. — The United States is about to begin destroying its largest remaining stockpile of chemical-laden artillery shells, marking a milestone in the global campaign to eradicate a debilitating weapon that still creeps into modern wars.

The Pueblo Chemical Depot in southern Colorado plans to start neutralizing 2,600 tons of aging mustard agent in March as the U.S. moves toward complying with a 1997 treaty banning all chemical weapons.