Defense Secretary Jim Mattis, whose primary public role in the fledgling Trump administration has been the reassurance of foreign allies, could stand to benefit from the exit of Michael Flynn, the president's former national security adviser, who the White House said was forced from his job by the president after an "erosion" of trust stemming from statements he made about a wiretapped conversation with Russian officials. Mattis, who was on a plane to Belgium Tuesday for a NATO conference when...