The defense chief has strongly defied his boss on torture, which Trump still maintains "absolutely" works. But despite the president's delusions about waterboarding and other enhanced interrogation techniques, Mattis' resistance appears to be winning out; Trump said he would let the defense secretary "override" him on the issue.

Mattis has butted heads with Trump on staffing. The Wall Street Journal reports a "degree of consternation" with Mattis from the White House, in part due to his continuing efforts to install Trump critics in the Defense Department. During Secretary of State Rex Tillerson's nomination, Mattis also reportedly was a strong advocate of the former CEO because Mattis viewed him as a potential partner against the more bellicose members of the Trump team, namely Flynn.

Mattis was also apparently annoyed about Trump's executive order banning travel from seven Muslim-majority countries. After it was implemented, he apparently was instrumental in getting Iraqis who helped U.S. troops receive exemptions, a policy that will reportedly continue in the revised order, due to be released next week. "Right now, I am assured that we will take steps to allow those who have fought alongside us, for example, to be allowed into the United States," Mattis told Iraqis Monday.

Mattis has also made it a point to reassure others allies around the world who are nervous about the president's antics.