Top U.S. military leaders pushed back Thursday against President Trump’s order—made in a series of three tweets—banning transgender people from serving in the Armed Forces. Marine Corps Gen. Joseph Dunford, the chair of the Joint Chiefs of Staff, said the announcement caught him by surprise and that the military would continue to allow transgender soldiers and officers to serve until Defense Secretary James Mattis received specific instructions from the president. This is Gen. Mark Milley, chief of staff of the Army, speaking Thursday.

Gen. Mark Milley: “To date, walking in here, I have yet to receive implementation guidance, implementation directives, from the Department of Defense, General Mattis. And we grow up and learn to obey the chain of command. And my chain of command is secretary of the Army and the secretary of defense—right?—and the president. So, we will work through the implementation guidance when we get it.”

Trump’s ban on transgender troops could impact as many as 15,000 people. It’s unclear whether they would face honorable or dishonorable discharges.