Mattis directly pointed to the response outside the White House on Monday when heavily armed officers used gas canisters and flash-bang grenades to clear Lafayette Square.

“When I joined the military, some 50 years ago, I swore an oath to support and defend the Constitution,” he wrote. “Never did I dream that troops taking that same oath would be ordered under any circumstance to violate the Constitutional rights of their fellow citizens — much less to provide a bizarre photo op for the elected commander-in-chief, with military leadership standing alongside.”