WASHINGTON — President Trump falsely asserted on Monday that his predecessor, Barack Obama, and other presidents did not contact the families of American troops killed in duty, drawing a swift, angry rebuke from several of Mr. Obama’s former aides. Mr. Trump was responding to a question about why he had not spoken publicly about the killing of four Green Berets in an ambush in Niger two weeks ago when he made the assertion. Rather than answering the question, Mr. Trump said he had written personal letters to their families and planned to call them in the coming week. Then he...