President Obama came face to face with the horror of nuclear war Friday in a somber visit to Hiroshima, becoming the first sitting U.S. president to tour the site of the atomic bombing 71 years ago that killed tens of thousands in an instant and ushered in the nuclear age. In a sweeping address that reflected on the obligations of humankind, Obama wrestled with the inherent contradiction that centuries of technical advancement have both made it easier to bind people together and given them the capacity for the carnage seen in this city. And he confronted the cold reality that...