President Donald Trump has officially granted the Department of Defense the legal authority to abolish the collective bargaining rights of its civilian labor unions representing some 750,000 workers.

Gutting the unions would provide “maximum flexibility,” Trump wrote in a memo published Thursday in the Federal Register, which was first reported by Government Executive.

Trump signed the memo three weeks ago, invoking “national security” to justify granting the Defense Department an exemption from the law giving all federal workers the right to unionize.

“When new missions emerge or existing ones evolve, the Department of Defense requires maximum flexibility to respond to threats to carry out its mission of protecting the American people,” Trump wrote in the memo. “Where collective bargaining is incompatible with these organizations’ missions, the Department of Defense should not be forced to sacrifice its national security mission.”

The 1978 Civil Service Reform Act contains a provision that allows a president to exclude agencies from engaging in collective bargaining with workers via written order in some circumstances, including “an emergency situation.”

Trump’s assault on unions contradicts his frequent claims to his base of supporting voters that he is a champion of the working class. A 2017 White House memo encouraged “eliminating employee unions” as part of a wide-ranging effort to weaken organized labor. Trump’s budget for fiscal 2021 would require federal workers to pay more for a cut in retirement benefits.

It’s not yet clear what Defense Secretary Mark Esper will do.

Labor leaders, workers and politicians have railed against the declaration.

“Denying ... Defense Department workers the collective bargaining rights guaranteed to them by law since 1962 would be a travesty — and doing it under the guise of ‘national security’ would be a disgrace,” Everett Kelley, national secretary-treasurer of the American Federation of Government Employees, said at a legislative conference earlier this month, according to The Washington Post. “This administration will not stop until it takes away all workers’ rights to form and join a union — and we will not stop doing everything we can to prevent that from happening.”