U.S. House of Representatives Speaker Nancy Pelosi of California on Friday took the unprecedented step of asking the chairman of the Joint Chiefs of Staff about "available precautions” to prevent President Donald Trump from initiating military action abroad or using his sole authority to launch nuclear weapons in the last days of his term.

In a phone call to the chairman, Gen. Mark A. Milley, Pelosi appeared to be seeking to have the Pentagon leadership essentially remove Trump from his authorities as the commander in chief. That could be accomplished by ignoring the president’s orders or slowing them by questioning whether they were issued legally.

But Milley appears to have made no commitments. Short of the Cabinet invoking the 25th Amendment or removing Trump through impeachment in the House and conviction in the Senate, it is unconstitutional to defy legal orders from the commander in chief.