Employees at Boeing’s St. Louis defense factories rejected a contract offered by the aerospace giant Sunday afternoon and around 3,200 workers began striking starting at midnight—the first walk out the company has faced at its St. Louis defense hub since 1996.

About 3,200 union members at Boeing facilities in Michigan and Illinois went on strike at midnight on Monday after rejecting an contract offer from the company, the union said.

About 3,200 workers at Boeing facilities voted to reject a modified four-year labor agreement with Boeing, the International Association of Machinists and Aerospace Workers union said Sunday.