When U.S. President Barack Obama and Japanese Prime Minister Shinzo Abe make a historic visit to Hiroshima on Friday - the first time a sitting U.S. president has visited the site of the first atomic bomb attack - their words advocating nuclear disarmament will clash with real-world security necessities. Far from backing up the vision of a world without nuclear bombs that Obama laid out in a 2009 speech that helped secure a Nobel Peace Prize, his near-finished presidency has seen a multibillion-dollar modernization of the U.S. nuclear force.