More than 3,200 workers at Boeing defense factories walked off the job Monday.

Members of the International Association of Machinists voted overwhelmingly to reject a tentative agreement that would have given union members a 20% pay increase over the next four years. The union did not detail why its members rejected the contract, saying it “fell short of addressing the priorities and sacrifices” of the union’s workers.

“3,200 highly-skilled IAM Union members at Boeing went on strike at midnight because enough is enough,” the union said in a statement on social media.

Workers on strike are from plants in St. Louis; St. Charles, Missouri; and Mascoutah, Illinois. They build military aircraft, including F-15 and F/A-18 fighter jets, the T-7A Red Hawk trainer, and the MQ-25 Stingray unmanned refueler. The St. Louis-area plants were also set to build the F-47 stealth fighter jet, a new-generation fighter.

“IAM District 837 members build the aircraft and defense systems that keep our country safe,” IAM Midwest Territory General Vice President Sam Cicinelli said in a statement Sunday. “They deserve nothing less than a contract that keeps their families secure and recognizes their unmatched expertise.”

Boeing CEO Kelly Ortberg said during the company’s earnings call last week that it will be able to deal with the cost of the strike, noting that this one was smaller than last year’s, in which 33,000 workers focused on commercial planes walked off the job for over two months.

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“The order of magnitude of this is much, much less than what we saw last fall,” he said. “I wouldn’t worry too much about the implications of the strike. We’ll manage our way through that.”

The strike marks another point in the company’s dramatic decade, having dealt with multiple fatal crashes and scandals in the last several years.