President Barack Obama will make history Friday as he becomes the first sitting U.S. president to visit the Japanese city of Hiroshima that was eviscerated by a U.S. atomic bomb during World War II. Obama will not apologize for the actions of his predecessors in Japan, he said, arguing that the trip was a way for former enemies to grow ever closer as allies.

"It's important to recognize that in the midst of war, leaders make all kinds of decisions, it's a job of historians to ask questions and examine them," Obama said. "But I know, as somebody who's now sat in this position for the last seven and a half years, that every leader makes very difficult decisions, particularly during wartime."

Long before Obama's arrival, the shadow of the bombing of Hiroshima loomed over his planned diplomatic visit. The bombings of Hiroshima and Nagasaki were the first and only time nuclear weapons have been used in warfare.

When U.S. President Harry Truman made the decision to drop twin bombs on Hiroshima and Nagasaki days apart in 1945, World War II had been raging since 1939 and the U.S. had been involved since 1941. While the U.S. had at first remained neutral in the European conflict, the U.S. declared war on Germany and its allies in 1941 after Japan bombed Pearl Harbor in Hawaii, killing 2,403 people.

Japan had refused to surrender after Germany accepted defeat in May 1945, and Truman argued that the complete destruction of Japanese cities was the only way to end the war that had killed millions of people. An atomic bomb tore through Hiroshima on Aug. 6, 1945, killing 80,000 people immediately and tens of thousands more from radiation. Three days later, a second bomb was dropped on Nagasaki, where 40,000 people died instantly and 10,000 more died from their injuries or from radiation. Japan surrendered less than 10 days later.

"We are now prepared to obliterate more rapidly and completely every productive enterprise the Japanese have above ground in any city,” Truman said as he announced the bombings to the U.S. in a televised address. “We shall destroy their docks, their factories and their communications. Let there be no mistake; we shall completely destroy Japan's power to make war."