what prompted this? >> reporter: our colleague who covers the pentagon and who has been all over this story today reports that president trump, according to her sources, was upset over this narrative that defense secretary jim mattis, the outgoing defense secretary, was seen as the last adult in the room, this notion that jim mattis was the last member of the cabinet standing who could reign in some of donald trump's more combative and isolationist impulses. this decision also follows the angry tweet from last night where he says when president obama fired jim mattis, i gave him a second chance. some thought i shouldn't. i thought i should. remember, jim mattis's resignation was seen as a clear rebuke of donald trump's world view and his actions to withdraw troops from syria and to order the pentagon to come up with a draw down plan for troops in afghanistan. so jim mattis will complete his tenure at the end of the year, the president says. stepping in is the deputy defense secretary patrick