Retired Marine Gen. James Mattis has been chosen to be secretary of defense by President-elect Donald Trump, according to people familiar with the decision. (Jenny Starrs/The Washington Post) By Dan Lamothe By Dan Lamothe December 1, 2016. President-elect Donald Trump said Thursday he has chosen retired Marine Gen. James N. Mattis, who has said that responding to “political Islam” is the major security issue facing the United States, to be secretary of defense.