Battles & Wars › World War II
Atomic bombings of Hiroshima and Nagasaki
August 6 and 9, 1945· Event· Asia & Pacific
Well documentedSolid records, physical evidence or scientific consensus.
The United States dropped atomic bombs on Hiroshima and Nagasaki in August 1945, the only nuclear weapons ever used in war.
See it on the Battles & Wars map →What happened
On August 6, 1945, the B-29 Enola Gay dropped a uranium bomb, 'Little Boy', on Hiroshima; on August 9 a plutonium bomb, 'Fat Man', hit Nagasaki. Each destroyed much of its city through blast, heat and fire, and radiation sickness killed more in the following months. Estimates vary, but by the end of 1945 roughly 90,000–166,000 had died in Hiroshima and 60,000–80,000 in Nagasaki, including thousands of Koreans, many of them forced laborers. Japan announced its surrender on August 15.
Why it matters
They helped end the war and opened the nuclear age; their horror underlies the taboo against using nuclear weapons that has held since 1945.
How we know
US strategic bombing surveys, Japanese medical and city records, decades of health studies of survivors (hibakusha), and the ruined Hiroshima Peace Memorial dome, now a UNESCO World Heritage site.
Still debated
Whether the bombs were necessary to force surrender, or whether Soviet entry and the blockade would have sufficed, remains one of history's most debated questions.
Inside this (1)
Connections (11)
▸Air power and strategic bombing reached its extreme in Atomic bombings of Hiroshima and NagasakiWell documented
The connection
Strategic bombing grew through the war: German raids on British cities, Allied raids on Hamburg and Dresden, and from March 1945 US firebombing that burned much of Tokyo and dozens of other Japanese cities. The atomic bombs carried this logic to its extreme: one plane, one bomb, one city.
When and how
Japan, 1945: B-29s of the US Twentieth Air Force, flying from the Mariana Islands, carried out both the firebombing campaign and the two atomic missions.
How we know
US targeting records show Hiroshima, Kokura and other candidate cities were deliberately left off firebombing lists so the new weapon's effect could be measured.
Debates and open questions
Whether the atomic bombs were a moral break with earlier bombing or its continuation is debated; the March 1945 Tokyo raid killed roughly as many as Hiroshima did immediately.
▸Battle of Okinawa shaped the invasion fears behind the Atomic bombings of Hiroshima and NagasakiMostly agreed
The connection
Okinawa's toll, tens of thousands of US casualties and enormous Japanese military and civilian deaths, shaped US projections for invading Japan's home islands (Operation Downfall). At a June 18, 1945 meeting, Truman said he hoped to prevent 'an Okinawa from one end of Japan to the other', a fear that weighed in using the bomb.
When and how
Washington, June–July 1945: Truman's meeting with the Joint Chiefs on June 18; the first atomic bomb fell on August 6.
How sure are we?
Minutes of the June 18 White House meeting and wartime casualty estimates for the invasion of Japan survive.
Debates and open questions
Postwar claims of half a million to a million American lives saved exceed most wartime estimates, which were lower; the figures remain contested.
▸Manhattan Project built the bombs used in Atomic bombings of Hiroshima and NagasakiWell documented
The connection
The project produced two bomb designs: a uranium 'gun' bomb (Little Boy), so reliable it was never tested, and a plutonium implosion bomb (Fat Man), tested at Trinity in July 1945. Bomb parts were shipped to Tinian in the Mariana Islands, where B-29 crews loaded them for the August missions.
When and how
Los Alamos, New Mexico, to Tinian Island, July–August 1945; the 509th Composite Group under Colonel Paul Tibbets flew the missions.
How we know
Project and military records, the Target Committee minutes and Truman's statements document the chain from design to use.
Debates and open questions
Whether Truman made a clear, deliberate decision, or largely followed momentum set by the project and the military, is debated by historians.
▸Atomic bombings of Hiroshima and Nagasaki hastened the end of World War IIMostly agreed
The connection
After Hiroshima, the Soviet declaration of war on August 8 and Nagasaki on August 9, Emperor Hirohito intervened to break a deadlock in Japan's leadership and accept surrender. His August 15 radio broadcast cited 'a new and most cruel bomb'. Japan formally surrendered on September 2, 1945.
When and how
Tokyo, August 9–15, 1945: the split Supreme War Council, the emperor's decision, and the formal surrender aboard USS Missouri on September 2.
How sure are we?
Japanese cabinet records, participants' testimony and the broadcast, which named the bomb; a rescript to the armed forces two days later stressed Soviet entry instead.
Debates and open questions
Historians such as Tsuyoshi Hasegawa argue Soviet entry mattered more than the bombs; others see the bombs as decisive. Most agree both played a part.
▸Atomic bombings of Hiroshima and Nagasaki shaped the logic of Nuclear weapons and deterrenceWell documented
The connection
Hiroshima and Nagasaki showed the world what nuclear weapons do to cities and people, killing well over 100,000 by the end of 1945; estimates vary. That horror made the threat of retaliation credible and actual use a taboo. No nuclear weapon has been used in war since.
When and how
Hiroshima on August 6 and Nagasaki on August 9, 1945; deterrence theory was developed afterward by strategists in the US and the Soviet Union.
How we know
Bernard Brodie wrote in 1946 that militaries' chief purpose must now be to avert wars; scholars such as Nina Tannenwald trace a 'nuclear taboo' back to the bombings.
Debates and open questions
Whether non-use since 1945 reflects deterrence, a moral taboo or luck is debated, as is whether the bombings were needed to end the war.
▸Strategic bomber delivered both bombs (B-29s) in the Atomic bombings of Hiroshima and NagasakiWell documented
The connection
Each atomic bomb weighed about 4.5 tonnes, and only the B-29 could carry one the roughly 2,400 km from Tinian to Japan. Specially modified B-29s of the 509th Composite Group flew the missions: Enola Gay dropped Little Boy on Hiroshima, and Bockscar dropped Fat Man on Nagasaki after clouds hid Kokura. Japan announced its surrender on August 15.
When and how
Tinian, Mariana Islands, to Hiroshima (August 6) and Nagasaki (August 9), 1945; Colonel Paul Tibbets and Major Charles Sweeney.
How we know
Mission logs, crew accounts, strike photographs, and American and Japanese damage surveys.
Debates and open questions
Whether the bombs or the Soviet declaration of war on August 8 did more to force Japan's surrender is debated by historians such as Tsuyoshi Hasegawa.
▸B-29 Superfortress program built the bombers that flew the Atomic bombings of Hiroshima and NagasakiWell documented
The connection
Specially modified 'Silverplate' B-29s were the only US planes able to carry the heavy atomic bombs that far. Flying from Tinian, Enola Gay dropped Little Boy on Hiroshima on August 6, 1945, and Bockscar dropped Fat Man on Nagasaki on August 9. Both planes were built by Martin in Omaha.
When and how
Tinian, Mariana Islands, August 1945; the 509th Composite Group under Colonel Paul Tibbets, with Bockscar flown by Major Charles Sweeney.
How we know
Mission records, crew accounts, and the two aircraft, now at the Smithsonian's Udvar-Hazy Center and the National Museum of the US Air Force.
Debates and open questions
The debate centers on the bombings themselves; the Smithsonian's planned 1995 Enola Gay exhibit was cut back after veterans protested its framing.
▸Mitsubishi: the Zero fighter and battleship Musashi saw its Nagasaki arms plants destroyed in the Atomic bombings of Hiroshima and NagasakiWell documented
The connection
Nagasaki was largely a Mitsubishi city: its shipyard, steel works, arms factory and torpedo plant employed tens of thousands. The bomb of August 9, 1945, exploded over the Urakami valley between the steel and arms works and the torpedo plant, destroying both. Many of those killed were Mitsubishi workers, including conscripted Korean laborers.
When and how
Nagasaki, August 9, 1945; the US B-29 Bockscar dropped the plutonium bomb 'Fat Man'.
How we know
US Strategic Bombing Survey reports, Japanese damage surveys and survivors' accounts document the plants' destruction.
Debates and open questions
Kokura was the first-choice target until clouds diverted the plane to Nagasaki; how many Korean workers died there is still uncertain.
▸Los Alamos Laboratory designed the two bombs used in the Atomic bombings of Hiroshima and NagasakiWell documented
The connection
Los Alamos designed both bombs. 'Little Boy' fired one piece of uranium into another inside a gun barrel and was considered so certain to work that it was never tested. 'Fat Man' used explosives to crush a plutonium core, a design proved at Trinity. Lab teams assembled both on Tinian.
When and how
Los Alamos, New Mexico, 1943–45; Trinity site, July 16, 1945; Tinian, August 1945; Oppenheimer's scientists and Project Alberta assembly teams.
How we know
Declassified design and assembly records, Project Alberta reports, and participants' memoirs.
Debates and open questions
Some project scientists, in Chicago's 1945 Franck Report, urged a demonstration on an empty area first; Oppenheimer's advisory panel saw no workable demonstration. Whether one could have worked is still debated.
▸Oak Ridge (Clinton Engineer Works) enriched the uranium used at Hiroshima in the Atomic bombings of Hiroshima and NagasakiWell documented
The connection
Oak Ridge's plants took natural uranium, less than 1% uranium-235, and step by step concentrated it to bomb grade, using thermal diffusion, gaseous diffusion and Y-12's calutrons in sequence. The roughly 64 kilograms of uranium in the Hiroshima bomb came from there.
When and how
Oak Ridge, Tennessee, 1944 – July 1945; the Y-12, K-25 and S-50 plants, run by Tennessee Eastman, Union Carbide and other contractors.
How we know
Declassified Manhattan District production records and General Groves's postwar account.
Debates and open questions
How much each plant contributed is hard to pin down, since material moved among all three before final enrichment.
▸Hanford Site (plutonium production) made the plutonium dropped on Nagasaki in the Atomic bombings of Hiroshima and NagasakiWell documented
The connection
Hanford's reactors turned uranium into plutonium, which chemical plants extracted and shipped to Los Alamos from early 1945. That plutonium filled the core of the Trinity test device on July 16 and of the Fat Man bomb dropped on Nagasaki on August 9, 1945.
When and how
Hanford, Washington, to Los Alamos, New Mexico, February–July 1945; DuPont operators and Army couriers.
How we know
Declassified Manhattan District production and shipping records.
Debates and open questions
Whether a second bomb was needed only three days after Hiroshima, before Japan's leaders had fully responded, is among the most debated questions of the war.