From Wikipedia

Days that Shook the World - Hiroshima

During the final stages of World War II in 1945, the United States conducted two atomic bombings against Japan in the cities of Hiroshima and Nagasaki.

After six months of intense strategic fire-bombing of 67 Japanese cities the Hirohito regime ignored an ultimatum given by the Potsdam Declaration. By executive order of President Harry S. Truman the U.S. dropped the nuclear weapon "Little Boy" on the city of Hiroshima on Monday, August 6, 1945, followed by the detonation of "Fat Man" over Nagasaki on August 9. These are the only attacks with nuclear weapons in the history of warfare.

Within the first two to four months of the bombings, the acute effects killed 90,000–166,000 people in Hiroshima and 60,000–80,000 in Nagasaki, with roughly half of the deaths in each city occurring on the first day. The Hiroshima prefectural health department estimates that, of the people who died on the day of the explosion, 60% died from flash or flame burns, 30% from falling debris and 10% from other causes. During the following months, large numbers died from the effect of burns, radiation sickness, and other injuries, compounded by illness. A plausible estimate of the total immediate and short term cause of death, 15–20% died from radiation sickness, 20–30% from flash burns, and 50–60% from other injuries, compounded by illness. Since then, more have died from leukemia (231 observed) and solid cancers (334 observed) attributed to exposure to radiation released by the bombs. In both cities, most of the dead were civilians.

Six days after the detonation over Nagasaki, on August 15, Japan announced its surrender to the Allied Powers, signing the Instrument of Surrender on September 2, officially ending the Pacific War and therefore World War II. Germany had signed its unavoidable Instrument of Surrender on May 7, ending the war in Europe. The bombings led, in part, to post-war Japan adopting Three Non-Nuclear Principles, forbidding the nation from nuclear armament.

The role of the bombings in Japan's surrender and the U.S.'s ethical justification for them is still debated.