1938: German scientists discover nuclear fission, which leads to the realization that a nuclear chain reaction could be created.
1942: The Manhattan Project is launched in the United States to develop a nuclear weapon, and research begins on both uranium and plutonium bombs.
1945: The first nuclear weapon, “Little Boy,” is dropped on Hiroshima, Japan, on August 6, killing around 140,000 people. Three days later, a second nuclear weapon, “Fat Man,” is dropped on Nagasaki, killing around 74,000 people.
1949: The Soviet Union tests its first nuclear bomb, ending the U.S. monopoly on nuclear weapons.
1952: The United States tests the first hydrogen bomb, which is many times more powerful than the atomic bombs used in World War II.
1957: The United States and Soviet Union agree to a nuclear testing moratorium.
1962: The Cuban Missile Crisis brings the world to the brink of nuclear war as the United States and Soviet Union engage in a tense standoff over Soviet nuclear missiles in Cuba.
1968: The Nuclear Non-Proliferation Treaty is signed, aiming to prevent the spread of nuclear weapons to non-nuclear states.
1970s-1980s: The United States and Soviet Union engage in a nuclear arms race, building up huge stockpiles of nuclear weapons.
1986: The Chernobyl nuclear disaster occurs in Ukraine, releasing a large amount of radioactive material and causing widespread health and environmental damage.
1991: The Strategic Arms Reduction Treaty is signed between the United States and Soviet Union, leading to significant reductions in their nuclear arsenals.
2020: The Treaty on the Prohibition of Nuclear Weapons enters into force, although it has not been signed by nuclear weapon states.