The Atomic Bomb

The concept of an atomic bomb appeared in science fiction years before it became a reality. Inspired by the work of atomic physicist Ernest Rutherford, novelist H. G. Wells described an all-powerful weapon he named an "atom bomb" in his 1914 sci-fi story The World Set Free.

In December 1938, physicists Otto Hahn and Fritz Strassman discovered a process that made the atomic bomb possible. Working in a laboratory in Berlin, Germany, the researchers were able to split atoms of the radioactive element uranium until a lighter element (barium) was formed.

The appearance of a different element initially confounded Hahn and other researchers, since turning one element into another was theoretically impossible. But in 1939, physicists Otto Frisch and his aunt Lise Meitner confirmed that Hahn and Strassman had successfully developed a process they named "fission."

In nuclear fission, the nucleus of an atom of radioactive material splits into two or more smaller nuclei, which causes a sudden, powerful release of energy. The discovery of nuclear fission opened up the possibility of nuclear technologies, including weapons.

Atomic bombs get their energy from fission reactions. Thermonuclear weapons, or hydrogen bombs, rely on a combination of nuclear fission and nuclear fusion. Nuclear fusion is another type of reaction in which two lighter atoms combine to release energy.

News about nuclear fission spread rapidly through the scientific community, along with concern about its use as a weapon, particularly given the rise of militant fascism in Germany and throughout Europe during the 1930s. In 1939, physicists Leo Szilard and Albert Einstein sent a letter to President Franklin D. Roosevelt warning of an "extremely powerful bomb."

Responding to fears that German scientists were working on a nuclear weapon, Roosevelt in 1942 authorized the formation of a nuclear research project bringing together various scientists and military officials. The Manhattan Project was the code name for this American-led effort to develop a functional atomic bomb during World War II.

Not all researchers were enthusiastic about the aims of the Manhattan Project. When Frisch asked Meitner to join him, she reportedly told her nephew, "I will have nothing to do with a bomb!"

The atomic bomb was invented under the leadership of physicist J. Robert Oppenheimer, often referred to as the “father of the atomic bomb." Dozens of other notable scientists including Szilard, Enrico Fermi, Ernest Lawrence and Edward Teller contributed to the project.

The secretive Manhattan Project was based primarily in Los Alamos, New Mexico. Additional work was conducted at Hanford, Washington, where stocks of radioactive plutonium were developed, and Oak Ridge, Tennessee, the site of uranium enrichment facilities.

On July 16, 1945, in a remote desert location near Alamogordo, New Mexico, Oppenheimer and his team successfully detonated the first atomic bomb in the Trinity Test. It created an enormous mushroom cloud some 40,000 feet high and ushered in the Atomic Age.

Scientists at Los Alamos had developed two distinct types of atomic bombs by 1945—a uranium-based design called “the Little Boy” and a plutonium-based weapon called “the Fat Man.”

While the war in Europe had ended in April 1945, fighting in the Pacific continued between Japanese forces and U.S. troops. In late July, President Harry Truman called for Japan’s surrender with the Potsdam Declaration. The declaration promised “prompt and utter destruction” if Japan did not surrender.

On August 6, 1945, the United States dropped its first atomic bomb from a B-29 bomber plane called the Enola Gay over the city of Hiroshima, Japan. The “Little Boy” exploded with about 13 kilotons of force, leveling five square miles of the city and killing 80,000 people instantly. Tens of thousands more would later die from radiation exposure.

When the Japanese did not immediately surrender, U.S. forces dropped a second atomic bomb three days later on the city of Nagasaki. The “Fat Man” killed an estimated 40,000 people on impact.

Citing the devastating power of “a new and most cruel bomb,” Japanese Emperor Hirohito announced his country’s surrender on August 14 (August 15 in Japan's time zone)—a day that became known as "V-J Day" and effectively ended World War II.

The United States, after successfully developing and using atomic weapons during the war, initially held a monopoly on this powerful new technology. However, the Soviet Union, driven by security concerns and a desire for international prestige, embarked on a determined effort to develop its own nuclear capabilities.

Within just a few years, the U.S.S.R. had obtained—through a network of spies engaging in international espionage—blueprints of a fission-style bomb and discovered regional sources of uranium in Eastern Europe. On August 29, 1949, the Soviets tested their first nuclear bomb, known as "First Lightning," at the Semipalatinsk Test Site in Kazakhstan.

In response to the Soviet's achievement, the United States initiated a program to develop thermonuclear weapons, which are significantly more powerful than the fission bombs used in World War II. The Cold War arms race had begun, and nuclear testing and research became high-profile goals for several countries, especially the United States and the Soviet Union.

Over the next few decades, both superpowers would stockpile tens of thousands of nuclear warheads. Other countries, including Great Britain, France and China, also developed nuclear weapons during this time.

To many observers, the world appeared on the brink of nuclear war in October 1962. The Soviet Union had installed nuclear-armed missiles on Cuba, just 90 miles from U.S. shores. This resulted in a 13-day military and political standoff known as the Cuban Missile Crisis.

President John F. Kennedy enacted a naval blockade around Cuba and made it clear the United States was prepared to use military force if necessary to neutralize the perceived threat.

Disaster was avoided when the United States agreed to an offer made by Soviet leader Nikita Khrushchev to remove the Cuban missiles in exchange for the United States promising not to invade Cuba.

The United States and Soviet Union took the lead in negotiating an international agreement to halt the further spread of nuclear weapons in 1968.

The Treaty on the Non-Proliferation of Nuclear Weapons (also called the Non-Proliferation Treaty or NPT) went into effect in 1970. It separated the world’s countries into two groups, nuclear weapons states and non-nuclear weapons states.

Nuclear weapons states included the five countries known to possess nuclear weapons at the time: the United States, the U.S.S.R., Great Britain, France and China.

According to the treaty, nuclear weapons states agreed not to use nuclear weapons or help non-nuclear states acquire nuclear weapons. They also agreed to gradually reduce their stockpiles of nuclear weapons with the eventual goal of total disarmament. Non-nuclear weapons states agreed not to acquire or develop nuclear weapons.

When the Soviet Union collapsed in 1991, there were still thousands of nuclear weapons scattered across Eastern Europe and Central Asia. Many of the weapons were located in Belarus, Kazakhstan and Ukraine. These weapons were deactivated and returned to Russia.

Many Americans became concerned about the health and environmental effects of nuclear fallout—the radiation left in the environment after a nuclear blast—in the wake of World War II and after extensive nuclear weapons testing in the Pacific during the 1940s and 1950s. Even prominent researchers including Oppenheimer and Szilard opposed further development of advanced nuclear weaponry.

The antinuclear movement emerged as a social movement in the 1950s at the height of the Cold War. During Women Strike for Peace demonstrations on November 1, 1961, co-organized by activist Bella Abzug, roughly 50,000 women marched in 60 cities in the United States to demonstrate against nuclear weapons.

This movement, often associated with the slogan "No Nukes," garnered widespread attention again in the 1970s and 1980s with high-profile protests against nuclear reactors after the Three Mile Island accident, a nuclear meltdown at a Pennsylvania power plant in 1979.

In 1982, a million people marched in New York City protesting nuclear weapons and urging an end to the Cold War nuclear arms race. It was one of the largest political protests in U.S. history.

Some countries wanted the option of developing their own nuclear weapons arsenal and never signed the NPT. India was the first country outside of the NPT to test a nuclear weapon in 1974.

Other non-signatories to the NPT include Pakistan, Israel and South Sudan. Pakistan has a known nuclear weapons program, and Israel is widely believed to possess nuclear weapons, though has never officially confirmed or denied the existence of a nuclear weapons program. South Sudan is not known or believed to possess nuclear weapons.

Some reports suggest that Iran, a signatory of the NPT, could be nine to 12 months away from being able to build a nuclear weapon. Following the U.S.-Israeli airstrikes on Iranian nuclear facilities in June 2025, the International Atomic Energy Agency (IAEA) withdrew its inspectors from Iran. As of 2026, the status of Iran's nuclear program remains unclear.

North Korea initially signed the NPT treaty, but announced its withdrawal from the agreement in 2003. Since 2006, North Korea has openly tested nuclear weapons, drawing sanctions from various nations and international bodies.

North Korea tested two long-range intercontinental ballistic missiles in 2017, including one reportedly capable of reaching the U.S. mainland. In September 2017, North Korea claimed it had tested a hydrogen bomb that could fit on top an intercontinental ballistic missile.

International observers agree that North Korea represents a real nuclear threat. In April 2026, a senior U.S. defense official testified that "North Korea’s nuclear forces are increasingly capable of targeting the U.S. Homeland, and its missile forces can strike South Korea and Japan with nuclear or conventional warheads. These forces are growing in size and sophistication, and they present a clear and present danger of nuclear attack on the American Homeland."

Pioneering Nuclear Science: The Discovery of Nuclear Fission. International Atomic Energy Agency. What is the Manhattan Project? Manhattan Project National Historical Park. 'Destroyer of Worlds': The Making of an Atomic Bomb. The National WWII Museum. The Development and Proliferation of Nuclear Weapons. NobelPrize.org. Here are the facts about North Korea’s nuclear test. NPR. North Korea’s Nuclear Weapons and Missile Programs. Congress.gov. Iran and Nuclear Weapons Production. Congress.gov.