The United States has destroyed the last of its declared chemical weapons stockpile, a milestone in the history of warfare dating back to World War I. Senate Republican leader Mitch McConnell on Friday said workers at the Blue Grass Army Depot in Kentucky eliminated the last of thousands of rockets filled with sarin nerve gas that have been stored there since the 1940s. It’s a watershed moment for the nation’s decadeslong effort to meet a September deadline for destroying weapons under the international Chemical Weapons Convention. Another Army site in Colorado completed destruction of its stockpile last month. Arms control advocates say they hope the ambitious project will be a model for ending stockpiles of other lethal weapons.