August 6, 1945: An atomic bomb is dropped on Hiroshima.

The “Little Boy” bomb was the second nuclear device detonated in history. “Little Boy”” measured three meters long and less than a meter wide. It weighed around 4,400 kilograms. And it killed over 140,000 people, 60,000 as a result of the initial blast. Weeks before the atomic bombings of Hiroshima and Nagasaki (and after the successful Trinity test), President Truman wrote in his diary:

We have discovered the most terrible bomb in the history of the world.

The president resolved that the bomb be used only on military targets and not on innocents, calling it both “the most useful” and “most terrible thing ever discovered”. The town of Hiroshima seemed to fit the bill, containing important military headquarters and depots of military supplies… and over 300,000 civilians. The Allies hoped, perhaps, that the detonation of this new horrific weapon and the killing of a hundred thousand innocent people would thoroughly disturb and wring out of the Japanese what the Potsdam Declaration had failed to - unconditional surrender. According to many Allied leaders (Truman and Churchill included), the dropping of the bomb saved millions of lives on both sides by making invasion of the mainland unnecessary, and it was also argued that the war should be ended as quickly as possible, before the Russians became involved. Americans, for the most part, reacted positively to the bombings (at first); according to polls, 85 percent believed that they had saved American lives, though as years passed and the scope of the tragedy came to light, the public’s perception soured. 

Herbert Hoover said that “the use of the atomic bomb, with its indiscriminate killing of women and children, revolts my soul”. Douglas MacArthur “saw no military justification” in its use; Chester Nimitz and Dwight Eisenhower also disputed the justification for the bombings, both believing that the Japanese would have surrendered regardless. Japanese surrender did come, anyway - eight days after Hiroshima and five days after Nagasaki.