President-elect Donald Trump is expected to nominate Gen. John Kelly for secretary of Homeland Security, according to two top transition officials familiar with the decision. Kelly was the commander of U.S. Southern Command until earlier this year. In that posting, he oversaw American military operations in South America and Central America. Gen. Kelly would become the second retired Marine general to join Mr. Trump’s cabinet. Both he and retired Marine Gen. James Mattis, the choice for defense secretary, must be confirmed by the Senate.