James N. Mattis

Reporting cited here

1. documented

   After Donald Trump selected James Mattis for secretary of defense, Mattis began serving on January 20, 2017.

   Mattis becomes Trump's first defense secretary · Served as defense secretary from January 2017 through December 2018

2. documented

   James Mattis submitted his resignation on December 20, 2018, writing that Trump should have a defense secretary whose views were better aligned with his; Defense Department history records Mattis's term ending December 31, 2018.

   Mattis resigns over differences with Trump · Former secretary of defense