FILE - In this Oct. 1, 2015 file photo, sailors of U.S. navy nuclear-powered aircraft carrier USS Ronald Reagan man the rails upon arrival as some U.S. flag-shaped balloons are hoisted to welcome them at the U.S. Navy's Yokosuka base in Yokosuka, south of Tokyo. Donald Trump has raised the specter of withdrawing U.S. forces from South Korea and Japan unless they share more of the burden of hosting the 80,000 troops, even as neighboring North Korea has conducted nuclear and missile tests with unprecedented intensity. (AP Photo/Eugene Hoshiko, File)