HIROSHIMA, Japan — Barack Obama on Friday paid tribute to the 140,000 people killed by the world’s first atomic bomb attack and sought to bring global attention to his unfulfilled vision of a world without nuclear weapons, as he became the first sitting U.S. president to visit Hiroshima. “Death fell from the sky and the world was changed,” Obama said, after laying a wreath, closing his eyes and briefly bowing his head before an arched monument in Hiroshima’s Peace Memorial Park that honors those killed on Aug.