Edit: Probably late on this, really late, but in case any other night shift anons missed it, Mattis threw in the towel and resigned.
Sauce: https://www.cnn.com/2018/12/20/politics/james-mattis-defense-secretary-resignation-reaction/index.html
Marco Rubio posted on Twitter:
Just read Gen. Mattis resignation letter. It makes it abundantly clear that we are headed towards a series of grave policy errors which will endanger our nation,damage our alliances & empower our adversaries. (1/2)
I hope we who have supported this administrations initiatives over the last two years can persuade the President to choose a different direction. But we must also fulfill our constitutional duty to conduct oversight over the policies of the executive branch. 2/2
Sauce: https://twitter.com/marcorubio/status/1075895147454439426
Pics are of the resignation letter.