President Obama arrived in Hiroshima on Friday, becoming the first sitting U.S. president to visit the site of the world’s first nuclear attack. The historic trip is taking place nearly 71 years after the bomb was dropped by an American B-29 warplane, killing about 140,000 people. Obama signed a guest book at the site, and wrote a message calling for courage to “spread peace and pursue a world without nuclear weapons.” Obama addressed a crowd inside a memorial park in Hiroshima. “We come to ponder the terrible force unleashed in the not so distant past,” he said. “Amongst those nations like my own that own nuclear stockpiles, we must have the courage to escape the logic of fear and pursue a world without them.” Aides said Obama will not apologize for the U.S. decision to drop the atomic weapon.