President Trump moved to replace the top ranks of the Department of Homeland Security on Monday, a day after forcing the resignation of its secretary, Kirstjen Nielsen, as he accelerated a purge of the nation’s immigration and security leadership. The White House announced the departure of Randolph D. Alles, the director of the Secret Service, who had fallen out of favor with the president even before a security breach at his Mar-a-Lago club that the agency effectively blamed on Mr. Trump’s employees. Government officials, who asked not to be identified discussing personnel changes before they were announced, said at least...