On August 6, 1945 at exactly 8:16AM ( Japan standard time) the worst thing that could happen did. A B-9 U.S bomber plane dropped an atomic nuclear bomb, in the heart of Japan in the city Hiroshima.
These bombs were intended to cause Japan to surrender to the allied forces and to bring about the end of WWII. They were warned what would happen if they didn't but they still chose not to.
Before the bombing the population for Hiroshima was 255,000. The aftermath of the bombing left 135,000 casualties; 66,000 deceased and 69,000 injured. This bombing wiped out 90% of the population.