| C. 540 BC | Democritus proposes atomism. |
| C. 440 BC | Aristotle attacks atomism. |
| C. AD 1200 | The Western world rediscovers atomism. |
| 1649 | French scientist Pierre Gassendi leads the revival of atomism with the publication of Syntagma philosophiae Epicuri. |
| 1803 | John Dalton of Britain proposes chemical atomic theory. |
| 1855–65 | The cathode-ray tube is invented and perfected. |
| 1895 | Wilhelm Conrad Röntgen of Germany discovers X-rays. |
| 1896–98 | Antoine Henri Becquerel and Pierre and Marie Curie of France discover natural radioactivity. |
| 1897 | J. J. Thomson of Britain discovers the electron. |
| 1899 | Ernest Rutherford of Britain discovers alpha and beta rays in his study of radium. |
| 1900 | Frederick Soddy of Britain is the first to identify and describe isotopes. |
| 1902 | Rutherford and Soddy publish the theory of radioactive decay. |
| 1905 | Einstein publishes the theory of relativity. (E=mc2). |
| 1911 | Rutherford discovers the nucleus of the atom. |
| 1913 | Niels Bohr of Denmark combines nuclear and quantum theory to create a new theory of atomic structure. |
| 1914 | H. G. Wells publishes The World Set Free and describes “atomic bombs.” |
| 1915 | Einstein publishes the general theory of relativity. |
| 1919 | Rutherford creates the first artificially induced nuclear reaction when he bombards nitrogen gas with alpha particles and transmutes it into an oxygen isotope. |
| 1929 | Ernest O. Lawrence of the United States develops the concept of the cyclotron to increase the speed of protons hurled at atomic nuclei. John Crockcroft and E. T. S. Walton develop the first linear accelerator for accelerating protons to study atomic transmutation. |
| 1931 | Ernest O. Lawrence builds his first cyclotron. |
| 1932 | James Chadwick of Britain discovers the neutron. |
| 1933 | Physicist Leó Szilárd is the first to realize that “a chain reaction might be set up if an element could be found that would emit two neutrons when it swallowed one neutron.” Frédéric and Irène Joliot-Curie of France discover artificial radioactivity. |
| 1934 | Enrico Fermi of Italy is the first to achieve nuclear fission in an experiment, but he does not realize his achievement. |
| 1938 | Otto Hahn of Germany conducts experiments that result in nuclear fission. |
| 1939 | Publication of Hahn’s results excites physicists around the world, who begin to conduct experiments in fission. Alarmed by the possibility of a uranium bomb, Albert Einstein writes to colleagues, who forward the letter to US President Franklin D. Roosevelt. Roosevelt responds by creating a committee to look into the military applications of atomic research. World War II begins. |
| 1940 | Scientists in the United Kingdom secretly encourage a British atomic bomb project. Using the cyclotron, scientists Philip Abelson and Edwin McMillan at the University of California, Berkeley, bombard uranium-238 to create “elements 93 and 94.” |
| 1941 | The United States enters the war. Scientist Glen Seaborg discovers that element 94 is plutonium. American scientists determine that it can be used to make an atomic bomb. The British “MAUD” Project determines that it is possible to make an atomic bomb with uranium-235. |
| 1942 | The United States creates the top-secret Manhattan Project to develop an atomic bomb. An experimental reactor “pile” at the University of Chicago generates the first self-sustaining nuclear reaction. |
| 1943 | The Manhattan Project establishes a top-secret city and laboratory at Los Alamos, New Mexico, to design and build the atomic bomb. Other secret facilities are built to create weapons-grade plutonium. |
| 1944 | As the US effort intensifies, German attempts to create an atomic bomb begin to lag. The race to develop the first atomic weapon is effectively over. |
| 1945 | The first atomic bomb is successfully tested at Trinity Site, New Mexico, on July 16. The second bomb is dropped on Hiroshima, Japan, on August 6. It is followed by a third bomb, which devastates Nagasaki, on August 9. Japan surrenders and World War II ends. |
| 1946 | The United States creates the Strategic Air Command (SAC) to deliver nuclear weapons in combat. US proposals for international control of atomic weapons stall in the United Nations (UN). In July, the US demonstrates and tests two atomic weapons at Bikini Atoll in the Marshall Islands. It also passes the Atomic Energy Act of 1946, creating the Atomic Energy Commission (AEC), which transfers control of weapons development from the military to the AEC. |
| 1947 | The United Kingdom authorizes the development of atomic weapons and begins its own, independent program under William Penney. The first British nuclear reactor is built. |
| 1948 | Under President Harry S. Truman, nuclear weapons development intensifies in the United States. The AEC is ordered to create an “Atomic Bomb Stockpile,” new laboratories and facilities are created, and Truman signs an order giving the President of the United States the sole decision to use atomic weapons in wartime. The United States continues atomic tests at Eniwetok Atoll in the Pacific. |
| 1949 | Top-secret Soviet efforts, aided by espionage, result in the first successful Soviet detonation of an atomic bomb at Semipalatinsk in Kazakhstan in August. The North Atlantic Treaty Organization (NATO) is formed, and the United States deploys nuclear-capable B-29s to the United Kingdom. |
| 1950 | The United States announces that it will develop the hydrogen bomb. |
| 1951 | The first postwar atomic tests in the continental United States take place in Nevada. American spies Julius and Ethel Rosenberg are sentenced to death for giving atomic secrets to the Soviets. |
| 1952 | The first British atomic bomb, “Hurricane,” is successfully detonated off western Australia in the Montebello islands on October 3. The same month, the United States detonates the first hydrogen bomb, a thermonuclear fusion device, “Mike,” at Eniwetok Atoll in the Pacific. |
| 1953 | The first Soviet attempt to create a hydrogen bomb is tested in Siberia. |
| 1954 | The most powerful surface detonation of an American hydrogen bomb vaporizes a small island at Bikini Atoll when the “Bravo” weapon yields 14.8 megatons and blankets more than a thousand square miles with highly radioactive fallout. |
| 1955 | The United Kingdom announces it will develop hydrogen bombs. Under Andrei Sakharov, the Soviets develop and successfully test their first true hydrogen bomb in November. |
| 1957 | The first British hydrogen bomb is successfully tested at Christmas Island in the Indian Ocean. The UN creates the International Atomic Energy Agency (IAEA). The Soviets announce they have successfully test-launched an intercontinental ballistic missile (ICBM). The prospect of nuclear deployment and attack by missile escalates the Cold War. The first US underground test of an atomic bomb takes place in Nevada. |
| 1958 | The United States and the United Kingdom agree to share nuclear weapons design for the first time since the end of World War II. The United States ends atmospheric tests of nuclear weapons and enters into an informal agreement with the United Kingdom and the Soviet Union to halt all tests. This moratorium lasts for nearly three years. |
| 1960 | The United States deploys its first operational ICBM, the Atlas D rocket, and the first American Polaris submarines, carrying nuclear missiles, enter service. |