The history of the atomic bomb is one of the most important and tragic chapters of the 20th century. Developed in secret during World War II, the bomb was used twice in August 1945, destroying the Japanese cities of Hiroshima and Nagasaki. Its existence changed warfare, international politics and the way humanity thinks about its own survival.
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The science behind the bomb
An atomic bomb releases energy through nuclear fission. When the nucleus of a heavy atom, such as uranium-235 or plutonium-239, is split by a neutron, it releases a large amount of energy and more neutrons. These neutrons can split other nuclei, creating a chain reaction. In a bomb, this chain reaction happens in a fraction of a second, producing an enormous explosion, intense heat and radiation.
Fission was discovered in 1938 by German scientists, and physicists quickly realized that it could be used to build a weapon of unprecedented power.
The Manhattan Project
In 1939, a letter signed by Albert Einstein, drafted with physicist Leo Szilard, warned U.S. President Franklin D. Roosevelt that Nazi Germany might develop an atomic bomb. This concern led to the creation of the Manhattan Project, a secret U.S. program with support from the United Kingdom and Canada.
The project involved tens of thousands of people and sites across the United States, including Oak Ridge, Tennessee; Hanford, Washington; and the Los Alamos laboratory in New Mexico, led by physicist J. Robert Oppenheimer.
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On July 16, 1945, the first nuclear explosion, known as the Trinity test, took place in the New Mexico desert.
Hiroshima and Nagasaki
On August 6, 1945, the United States dropped a uranium bomb, nicknamed “Little Boy,” on Hiroshima. Three days later, on August 9, a plutonium bomb called “Fat Man” was dropped on Nagasaki.
The destruction was immense. Tens of thousands of people died instantly, and many more died in the following weeks and months from burns, injuries and radiation sickness. Estimates indicate that, by the end of 1945, the total number of deaths in the two cities exceeded 200,000. Survivors, known in Japan as hibakusha, faced long-term health problems and social discrimination.
Japan announced its surrender on August 15, 1945, and World War II ended officially on September 2.
The nuclear arms race
The atomic age did not end with the war. The Soviet Union tested its first atomic bomb in 1949, beginning a nuclear arms race during the Cold War. Both superpowers developed hydrogen bombs, which use nuclear fusion and are far more powerful than the bombs used in 1945. The largest explosion ever, the Soviet “Tsar Bomba” test in 1961, was thousands of times more powerful than the Hiroshima bomb.
The concept of “mutually assured destruction” emerged: since each side could destroy the other, a direct nuclear war would be suicidal. Crises such as the Cuban Missile Crisis in 1962 showed how close the world came to disaster.
Efforts to limit nuclear weapons
Over the decades, countries signed agreements to reduce the risks:
- Partial Test Ban Treaty (1963): banned nuclear tests in the atmosphere, in space and underwater.
- Nuclear Non-Proliferation Treaty (1968): aims to prevent the spread of nuclear weapons, promote disarmament and allow peaceful uses of nuclear energy.
- Arms reduction agreements between the United States and the Soviet Union, later Russia, which reduced their arsenals from Cold War peaks.
- Treaty on the Prohibition of Nuclear Weapons: entered into force in 2021 for the countries that joined it.
In 2024, the Japanese organization of atomic bomb survivors, Nihon Hidankyo, received the Nobel Peace Prize for its work toward a world without nuclear weapons.
The long-term impacts
- Health: survivors faced higher risks of certain cancers and other illnesses.
- Politics: nuclear weapons became a central element of global power and diplomacy.
- Culture: the bomb influenced literature, cinema and art, and became a symbol of humanity’s destructive potential.
- Science and energy: nuclear research also led to peaceful applications, such as nuclear power plants and medical treatments.
Nuclear weapons today
Several countries possess nuclear weapons. According to estimates by independent research organizations such as the Stockholm International Peace Research Institute (SIPRI) and the Federation of American Scientists, the global stockpile is much smaller than at the peak of the Cold War, when tens of thousands of warheads existed, but thousands remain. The United States and Russia hold the vast majority. Exact numbers are not publicly confirmed by all governments, which is why researchers present them as estimates.
Hibakusha: the survivors
The survivors of the Hiroshima and Nagasaki bombings are known in Japan as hibakusha. Many suffered severe burns, injuries and radiation-related illnesses, as well as discrimination and psychological trauma. For decades, hibakusha have shared their testimonies to warn the world about the consequences of nuclear weapons. In 2024, the Japanese organization Nihon Hidankyo, formed by survivors, received the Nobel Peace Prize for its efforts to achieve a world free of nuclear weapons.
Peace memorials
The Hiroshima Peace Memorial, known as the Atomic Bomb Dome, is a UNESCO World Heritage Site. It is one of the few structures left standing near the hypocenter and was preserved as a reminder of the destruction. The Hiroshima Peace Memorial Museum and the Nagasaki Atomic Bomb Museum receive visitors from around the world, and annual ceremonies in both cities call for peace and nuclear disarmament.
Nuclear energy vs. nuclear weapons
The same physics behind the atomic bomb, nuclear fission, is used in nuclear power plants to generate electricity in a controlled way. Nuclear energy produces large amounts of electricity with low carbon emissions, but it raises concerns about safety, radioactive waste and the risk that technology or materials could be diverted to weapons. International safeguards, such as inspections by the International Atomic Energy Agency (IAEA), aim to prevent this.
Ethical debates
Historians and ethicists continue to debate the decision to use atomic bombs in 1945. Some argue that it hastened the end of the war; others emphasize the massive civilian casualties and question whether alternatives existed. Beyond historical debate, the bombings raise lasting questions about the ethics of weapons capable of destroying entire cities and about humanity's responsibility to prevent their use.
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Conclusion
The history of the atomic bomb shows both the extraordinary power of science and the terrible consequences of using it for destruction. The suffering in Hiroshima and Nagasaki remains a warning for the world. Remembering this history is essential to support efforts that prevent nuclear weapons from ever being used again.