The strike started at Boeing facilities in St. Louis; St. Charles, Missouri; and Mascoutah, Illinois, after about 3,200 local members of the International Association of Machinists and Aerospace Workers voted Sunday to reject a modified four-year labor agreement, the union said.

Roughly 3,200 machinists at Boeing’s defense hub in the St. Louis area went on strike Monday, the first time the union has done so in nearly three decades, after it rejected the latest contract offer from the aerospace giant.