Donald Trump’s defense secretary pick was not involved in the murder of detainees, but the formative experience influenced his advocacy for troops accused of war crimes.

When President-elect Donald Trump chose the television commentator Pete Hegseth as his nominee for secretary of defense, a panoply of national security stalwarts — retired generals and admirals, former appointees, and elected officials — evinced surprise.

President-elect Donald J. Trump’s pick to lead the Defense Department was described by peers as a skillful soldier who became embittered by military dysfunction.