An Iranian assassination threat against President Donald Trump prompted an extraordinary operation last month in which he flew secretly from Turkey on an alternate military aircraft while the White House said he was aboard Air Force One, The Washington Post has learned. The clandestine mission, which has not been reported previously, was carried out unbeknownst to journalists and some White House staff members who believed they were on the same plane as the president, according to material reviewed by The Post, a U.S. official familiar with the operation and another person with knowledge of the president’s travel. These people spoke...