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Atomic Age Timeline: Key Events and History

From the discovery of nuclear fission to the Cold War arms race, the events that opened the nuclear era.

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1938 Nuclear fission discovered 1942 Manhattan Project and first chain reaction 1945 Trinity test 1945 Hiroshima and Nagasaki bombed 1949 Soviet Union tests atomic bomb 1952 First hydrogen bomb tested 1991 Cold War ends

1938 Nuclear fission is discovered in Berlin

Otto Hahn and Fritz Strassmann's experiments, interpreted by Lise Meitner and Otto Frisch, reveal that uranium atoms can be split, releasing large amounts of energy.

1942 The Manhattan Project achieves the first controlled chain reaction

The US-led Manhattan Project produces the first self-sustaining nuclear chain reaction under a stadium at the University of Chicago in December 1942.

1945 The first nuclear weapon is detonated at Trinity

The United States detonates the world's first nuclear device in the New Mexico desert on July 16, 1945.

1945 Atomic bombs are dropped on Hiroshima and Nagasaki

US bombers drop atomic bombs on the Japanese cities on August 6 and August 9, 1945, the only use of nuclear weapons in warfare to date.

1949 The Soviet Union tests its first atomic bomb

The USSR's successful test ends the US nuclear monopoly and begins the Cold War nuclear arms race.

1952 The United States tests the first hydrogen bomb

The 'Ivy Mike' test in the Pacific demonstrates a thermonuclear weapon far more powerful than earlier atomic bombs; the Soviet Union follows with its own test within a few years.

1991 The Cold War ends amid arms-reduction efforts

The collapse of the Soviet Union in 1991 follows a period of arms-control treaties that begin to reduce the vast nuclear arsenals built up over the preceding decades.

Last updated: August 2026

History Timelines. (August 2026). Atomic Age Timeline: Key Events and History. Retrieved from https://history-timelines.org.uk/events/atomic/

This timeline of the Atomic Age traces the path from the 1938 discovery of nuclear fission through the Manhattan Project, the atomic bombings of Japan, and the Cold War arms race that followed.

Discovery and the Manhattan Project, 1938–1945

The atomic age opened in Berlin in late 1938, when chemists Otto Hahn and Fritz Strassmann observed that bombarding uranium with neutrons produced barium, an unexpected result. Physicists Lise Meitner and Otto Frisch, working in exile in Sweden, correctly interpreted this as nuclear fission: the splitting of an atomic nucleus, which releases substantial energy. The discovery, combined with fears that Nazi Germany might develop a weapon based on the principle, prompted scientists including Albert Einstein to alert the US government, eventually leading to the establishment of the Manhattan Project, a large, secret Allied research and development program. The project achieved a key milestone in December 1942, when a team led by physicist Enrico Fermi produced the first self-sustaining nuclear chain reaction beneath the stands of a stadium at the University of Chicago, proving that a controlled reaction was possible. The Manhattan Project ultimately grew into a vast enterprise spanning multiple secret sites across the United States, including Los Alamos, New Mexico, where the weapon itself was designed, and Oak Ridge, Tennessee, and Hanford, Washington, where the necessary fissile material was produced, employing well over 100,000 people at its peak, most of whom had no knowledge of the project’s ultimate purpose.

A close-up of an antique scientific instrument panel with dials

Trinity, Hiroshima, and Nagasaki, 1945

The Manhattan Project culminated on July 16, 1945, when the United States detonated the world’s first nuclear device at the Trinity test site in the New Mexico desert, confirming that a functional atomic weapon had been built. Less than a month later, with Japan still fighting on despite Germany’s defeat, US bombers dropped atomic bombs on the Japanese cities of Hiroshima on August 6 and Nagasaki on August 9, 1945. The bombings caused immense destruction and loss of life; death toll estimates vary and are generally cited in ranges of tens of thousands in each city by the end of 1945, with the total rising in subsequent years due to radiation-related illness, though precise figures remain a subject of historical study. Japan announced its surrender within days, and the bombings remain, as of this writing, the only use of nuclear weapons in armed conflict. The decision to use the bombs, and its role in ending the war relative to other factors such as the Soviet declaration of war on Japan in early August 1945, has remained a subject of significant historical debate ever since.

The Cold War Arms Race, 1949–1991

The US monopoly on nuclear weapons ended in 1949, when the Soviet Union successfully tested its own atomic bomb, sooner than many Western analysts had expected. The two superpowers then entered a prolonged arms race, developing progressively more powerful weapons; the United States tested the first hydrogen, or thermonuclear, bomb in 1952, a device far more destructive than the bombs used in 1945, and the Soviet Union tested a comparable weapon within a few years. Over the following decades, both nations built vast nuclear arsenals and delivery systems, a buildup punctuated by moments of acute danger, such as the 1962 Cuban Missile Crisis, alongside periodic efforts at arms control, including test-ban and strategic arms limitation treaties. The nuclear rivalry that defined much of the Cold War eased gradually from the late 1980s onward and effectively concluded with the dissolution of the Soviet Union in 1991, though the weapons and strategic doctrines developed during this period continue to shape international security into the present day. Other nations also developed nuclear arsenals during and after this period, including the United Kingdom, France, and China by the 1960s, and, in the following decades, additional states. This reflects the way nuclear weapons capability, once demonstrated, proved difficult to confine to the original wartime powers, and remains a live issue in global diplomacy today.