Keep in mind that in the latter days of the Watergate crisis, Defense Secretary Schlesinger and Secretary of State Kissinger kept President Nixon from direct access to the military, especially the Marines, whose commandant was sympathetic to the President's plight. By that time, however, Nixon had lost most of his confidants like Haldemann and Erlichman, was depressed, and drinking heavily. It is unlikely Trump has Nixon's problems, but would Defense Secretary Mattis do what Schlesinger and Kissinger did 42 years ago, thus blocking Trump from using the military to decapitate the hostile Federal spy and law enforcement agencies?
“...possibly a warning to the Deep State that the military could seize Langley and the J. Edgar Hoover building under martial law.”
I offer no proof but lean towards that theory. Also, consider me a biased observer of these organizations.
Long ago, the CIA dumped one of their operatives into our unit. After we determined that he was a nitwit and might be detrimental to the mission, the gunny called the local mama-san. She took him in hand to the local village where he drank himself silly and probably contracted a half-dozen different social diseases over a period of several months.