Milley did what Al Haig did in 1981. He usurped the entire Chain of Command and essentially declared himself "in control of the American military".
Milley convened meetings with top military staff and informed them that although Trump was still 'technically' President, he (Milley) was the one calling the shots, and the only one allowed to give military directives and orders. Then he called on each one at the meeting to affirm his, and his alone, authority.