Homeland Security Secretary John Kelly’s move to White House chief of staff is ushering in a cloud of uncertainty at the department that has led some of President Donald Trump’s most controversial undertakings — from its roundup of undocumented immigrants to his travel ban and proposed border wall. It may also set the stage for a brutal confirmation fight if President Donald Trump tries to replace the retired Marine general with an outspoken opponent of illegal immigration instead of a seasoned bureaucrat or lawmaker. A person close to Kelly said it's unclear who will replace him. But several White House...