http://time.com/4346336/atomic-bombs-1945-history/
We may debate the morality of the choice, but history can show why American officials would have thought the bomb was necessary
President Obama’s visit to Hiroshima, nearly 71 years after it was destroyed by the first atomic bomb, inevitably raises once again the questions of why the United States dropped that bomb, whether it was necessary to convince Japan to surrender and whether it saved lives by making it unnecessary to invade the Japanese home islands.
Beginning in the 1960s, when the Vietnam War disillusioned millions of Americans with the Cold War and the U.S. role in the world, the idea that the bombing of Hiroshima—and the subsequent bombing of Nagasaki—was not necessary gained ground. Led by the economist Gar Alperovitz, a new school of historians also began arguing that the bomb was dropped more to intimidate the Soviet Union than to defeat the Japanese. By 1995, Americans divided so sharply on the necessity and morality of dropping the bombs that a 50th anniversary exhibit at the Smithsonian had to be repeatedly altered and eventually drastically scaled back. Passions have cooled as the generation that fought the war has left the scene and academics have turned to other topics, but the President’s visit is bound to reignite them..."