WASHINGTON — President-elect Donald J. Trump has settled on Gen. John F. Kelly, a retired four-star Marine general whose son was killed in combat in Afghanistan, as his choice for secretary of homeland security, placing defense of American territory from terrorism in the hands of a seasoned commander with personal exposure to the costs of war.

General Kelly, 66, who led the United States Southern Command, had a 40-year career in the Marine Corps, and led troops in intense combat in western Iraq. In 2003, he became the first Marine colonel since 1951 to be promoted to brigadier general while in active combat.

Mr. Trump, a person briefed on the decision said, has not yet formally offered the job to General Kelly, in part because the general is out of the country this week. The president-elect plans to roll out the appointment next week, along with his remaining national security positions, including secretary of state.