Jim Mattis has been trying to put the brakes on some of Trump's more heinous policies.

"White House officials said Mattis had balked at a number of Trump’s requests. That included initially slow-walking the president’s order to ban transgender troops from the military and refusing a White House demand to stop family members from accompanying troops deploying to South Korea. The Pentagon worried that doing so could have been seen by North Korea as a precursor to war."

Over the last four months alone, the president and the defense chief have found themselves at odds over NATO policy, whether to resume large-scale military exercises with South Korea and, privately, whether Trump’s decision to withdraw the United States from the Iran nuclear deal has proved effective.
Link: Fraying Ties With Trump Put Mattis’ Fate in Doubt
[tampabay.com]