Fear: Trump in the White House by Bob Woodward:
White House Press Secretary Sean Spicer, who was a commander in the Naval Reserves, tried several times to persuade Mattis to appear on Sunday talk shows on behalf of the administration. The answer was always no.
“Sean,” Mattis finally said, "I’ve killed people for a living. If you call me again, I’m going to fucking send you to Afghanistan. Are we clear?”