The U.S. Army is set to begin destroying the nation's largest remaining stockpile of chemical weapons.

Pueblo Chemical Depot in Colorado plans to begin eliminating the first of 780,000 shells containing 2,600 tons of mustard agent on Wednesday if the weather is favorable for moving the material.

Most of the weapons will be neutralized in a highly automated $4.5 billion plant during the next four years.

About 1,400 damaged shells will be destroyed first. They're considered unsuitable for the main plant, so they'll be torn open with explosives inside a sealed chamber, and the mustard will be chemically neutralized.

Another 523 tons of mustard and nerve agents are stored at Blue Grass Army Depot in Richmond, Kentucky. Destruction of that stockpile will start in 2016 or 2017.

The weapons are being destroyed under a 1997 treaty.