Timeline of nuclear weapons development

Before 1930

• 1895 – Wilhelm Konrad Röntgen discovers X-rays at the University of Würzburg.
• 1896 – Henri Becquerel discovers that uranium emits radiation at the National Museum of Natural History in Paris.
• 1898 – J.J. Thomson observes the photoelectric effect.
• 1900 – Max Planck theorizes that matter can only absorb energy in fixed quanta.
• 1904 – Frederick Soddy first proposes a bomb powered by nuclear fission to the Royal Engineers.
• 1905 – Albert Einstein proposes mass–energy equivalence, deriving from his theory of special relativity, which he had developed earlier that year.
• 1911 – Ernest Rutherford discovers that the majority of the energy in an atom is contained in the nucleus through experiments at the University of Manchester.
• 1912 – J.J. Thomson discovers isotopes through experiments with neon.
• 1914 – H.G. Wells writes The World Set Free, a science fiction novel postulating a world war in 1956 pitting the United Kingdom and France against Germany and Austria-Hungary. Inspired by the research of Rutherford, Sir William Ramsay, and Frederick Soddy, the novel predicts the development of atomic weapons, and features a "carolinum"-based hand grenade that does not extinguish once detonated.
• 1920 – Rutherford postulates the existence of a neutral particle in the atomic nucleus at a Bakerian Lecture in London.
• 1924 – Writing for The Pall Mall Gazette, Winston Churchill speculates "Might a bomb no bigger than an orange be found to possess a secret power to destroy a whole block of buildings – nay to concentrate the force of a thousand tons of cordite and blast a township at a stroke?"

  1930–1940

• 1932 – James Chadwick discovers the neutron, leading to experiments in which elements are bombarded with the new particle.
• 1933 – Leó Szilárd realizes the concept of the nuclear chain reaction, although no such reaction was known at the time. He invented the idea of an atomic bomb in 1933 while crossing a London street in Russell Square. He patented it in 1934.
• 1934 – Enrico Fermi conducts experiments in which he exposes uranium and thorium to neutrons to create distinct new substances. Although he is unaware at the time, he creates the first synthetic elements, the transuranium elements.
• 1938 – Fermi is awarded the Nobel Prize in Physics for his achievements, and flees from Fascist Italy to the United States due to the racial laws ratified under pressure from Nazi Germany.
• 1938 – December – The German chemists Otto Hahn and Fritz Strassman detect barium after bombarding uranium with neutrons. This is correctly interpreted by Lise Meitner and her nephew Otto Robert Frisch as nuclear fission.
• 1939 – January – Otto Robert Frisch experimentally confirms Otto Hahn and Fritz Strassman's discovery of nuclear fission. Frisch goes to Copenhagen to share the discovery with Niels Bohr, who in turn reports the discovery to his American colleagues. Bohr and John Archibald Wheeler determine later that year through chain-reaction experiments at Princeton University that uranium-235 could produce a nuclear explosion.
• 1939 – April – Nazi Germany begins the German nuclear energy project.
• 1939 – September 1 – World War II begins after the invasion and subsequent partition of Poland between Nazi Germany and the Soviet Union.
• 1939 – October – U.S. President Franklin D. Roosevelt receives the Einstein–Szilárd letter and authorizes the creation of the Advisory Committee on Uranium. The Uranium Committee has its first meeting on October 21, and $6,000 was budgeted for conducting neutron experiments.

  1940–1950

• 1940 – April – The MAUD Committee is established by Henry Tizard and the British Ministry of Aircraft Production to investigate feasibility of an atomic bomb.
• 1940 – May – The paper which Dr. Yoshio Nishina of Nuclear Research Laboratory of Riken and Professor of Chemical Institute, Faculty of Science, Imperial University of Tokyo, Kenjiro Kimura presented to Physical Review, showed that they had produced neptunium-237 by exposing triuranium octoxide to fast neutrons for more than 50 hours.
• 1940 – May - After the defeat of Belgium in only 18 days, the Nazis took possession of a significant amount of high quality uranium ore from the Belgian Congo, some still "on the docks". In 1939 both Britain and France had expressed interest in securing Belgium's uranium inventory but no action was taken.
• 1940 - June - The French Third Republic collapses during the Battle of France. The rapid military collapse would contribute to nearly universal French public support for a nuclear deterrent in later years.
• 1940 – July – The paper explaining that Dr. Yoshio Nishina and Kenjiro Kimura discovered symmetric fission on the previously described test appeared in Nature. The LibreTexts libraries based upon work supported by the National Science Foundation says, "Multiple combinations of symmetric fission products are possible for fission chain reactions." And, again, it as fission product yield, is known that the higher the energy of the state that undergoes nuclear fission is more likely a symmetric fission.
• 1940 – July – The Soviet Academy of Sciences starts a committee to investigate the development of a nuclear bomb.
• 1940 – September – Belgian mining engineer Edgar Sengier orders that half of the uranium stock available from the Shinkolobwe mine in the Belgian Congo—about 1,050 tons—be secretly dispatched to New York by African Metals Corp., a commercial division of Union Minière.
• 1941 – February – Plutonium discovered by Glenn Seaborg and Arthur Wahl at the University of California, Berkeley.
• 1941 – May – A review committee postulates that the United States will not isolate enough uranium-235 to build an atomic bomb until 1945.
• 1941 – June – President Roosevelt forms the Office of Scientific Research and Development under Vannevar Bush.
• 1941 – June 15 – The MAUD Committee approves a report that a uranium bomb could be built.
• 1941 – June 22 – Operation Barbarossa, the 1941 German invasion of the Soviet Union, begins. Soviet nuclear research is subsequently delayed.
• 1941 – October – President Roosevelt receives MAUD report on the design and costs to develop a nuclear weapon. Roosevelt approves project to confirm MAUD's finding.
• 1941 – December – The United States enters World War II after the Pearl Harbor attack and the German declaration of war against the United States, leading to an influx in funding and research for atomic weapons.
• 1942 – The United Kingdom opts to support the United States' efforts to build a bomb rather than to pursue its own nuclear weapons program due to wartime economic damage, and allows the Tube Alloys programme to be subsumed into the American project.
• 1942 – April – Joseph Stalin was first informed of the efforts to develop nuclear weapons based on a letter sent to him by Georgii Flerov pointing out that there was nothing being published on nuclear fission since its discovery, and the prominent physicists likely involved had not been publishing at all. This urged the Soviet Union to start a nuclear weapons program.
• 1942 – July – The Heereswaffenamt relinquishes control of the German nuclear energy project to the Reichsforschungsrat, essentially making it only a research project with objectives far short of making a weapon.
• 1942 – July through September – A summer conference at University of California, Berkeley is convened by physicist Robert Oppenheimer and discusses the design of a fission bomb. Edward Teller introduces the "Super" hydrogen bomb as a major discussion point.
• 1942 – August through November – The Manhattan Project is established by the U.S. Army Corps of Engineers under command of General Leslie Groves. "Site X" is chosen in Tennessee, for isotopic separation of uranium-235 from natural uranium, and will later become Oak Ridge National Laboratory. Hanford Site is chosen in Washington, for making plutonium in nuclear reactors. "Site Y" is chosen by Groves and Dr. J. Robert Oppenheimer near Albuquerque, New Mexico, for bomb design and manufacture, and will later become Los Alamos National Laboratory.
• 1942 – September - Lieutenant Colonel Kenneth Nichols meets Edgar Sengier in the New York offices of Union Minière. Nichols has been ordered by General Groves to find uranium. Sengier's answer has become history: "You can have the ore now. It is in New York, a thousand tons of it. I was waiting for your visit." Nichols reaches an agreement with Sengier that an average of 400 tons of uranium oxide will begin shipping to the US from Shinkolobwe each month.
• 1942 – October - 100 tons of Sengier's uranium ore is sent to Canada for refining by Eldorado Mining and Refining in Port Hope, Ontario.
• 1942 – October - A special detachment from United States Army Corps of Engineers arrives in the Belgian Congo to reopen the Shinkolobwe mine. Work involves draining water from flooded workings, upgrading the plant machinery and constructing transportation facilities.
• 1942 – November - The first uranium oxide shipment leaves the Congolese port of Lobito. Only two shipments will ever be lost at sea. Aerodromes at Elizabethville and Leopoldville are expanded with US assistance. The OSS is employed to prevent ore smuggling to Nazi Germany.
• 1942 – December 2 – Enrico Fermi and his team achieve the first controlled nuclear reaction at Chicago Pile-1 constructed at the University of Chicago in a squash court underneath Stagg Field.
• 1943 – Laboratory No. 2 is established to pursue nuclear weapons research under Igor Kurchatov.
• 1943 – March – The Japanese Committee on Research in the Application of Nuclear Physics, chaired by Yoshio Nishina concludes in a report that while an atomic bomb was feasible, it would be unlikely to produce one during the war. Japan then concentrated on research into radar.
• 1943 – April – Introductory lectures begin at Los Alamos, which later are compiled into The Los Alamos Primer.
• 1943 – August – The Quebec Agreement is signed by President Roosevelt and British Prime Minister Winston Churchill. James Chadwick, as head of the British Mission to the Los Alamos Laboratory, led a multinational team of distinguished scientists that included Sir Geoffrey Taylor, James Tuck, Niels Bohr, Rudolf Peierls, Otto Frisch, Klaus Fuchs and Ernest Titterton.
• 1944 – April – Emilio Segrè discovers that the spontaneous fission rate of plutonium is too high to be used in a gun-type fission weapon. Leads to change in priority to the design of an implosion-type nuclear weapon. The calutrons at the Y-12 uranium enrichment plant are activated.
• 1944 – July – Sergei Korolev is released from a Gulag and assigned for rocket development.
• 1944 – September – The first plutonium reactor is activated in Hanford, but shuts itself off immediately.
• 1944 – September 8 – The Wehrmacht launches the V-2 rocket, the first ballistic missile and the template for later American and Soviet nuclear missile designs. It is based on the designs of Wernher von Braun.
• 1945 – March 10 – A Japanese Fu-Go balloon bomb nearly knocks out electrical power to the Hanford plant.
• 1945 – April 12 – U.S. Vice President Harry S. Truman is inaugurated President after the death of Franklin D. Roosevelt, and is informed about the Manhattan Project by War Secretary Henry L. Stimson.
• 1945 – May – The United States captures a number of important German rocket scientists, including Wernher von Braun, for work on American missile programs through Operation Paperclip. Von Braun is eventually assigned to the Army Ballistic Missile Agency at the Redstone Arsenal in Huntsville, Alabama.
• 1945 – June – The Office of Military Government, United States hands over Nordhausen, including the Mittelwerk factory where the V-2 rocket was constructed, to the Group of Soviet Occupation Forces in Germany. Soviet forces find documents and equipment from the factory and recruit Helmut Gröttrup.
• 1945 – July 16 – The first nuclear explosion, the Trinity test of an implosion-type plutonium-based nuclear weapon known as "the gadget", near Alamogordo, New Mexico.
• 1945 – July 22 – Truman alludes to Stalin about having successfully detonated an atomic bomb at the Potsdam Conference.
• 1945 – August 6 – "Little Boy", a gun-type uranium-235 weapon, is dropped on Hiroshima, Japan.
• 1945 – August 9 – "Fat Man", an implosion-type plutonium-239 weapon, is dropped on Nagasaki, Japan.
• 1945 – August 11 – The Smyth Report is published detailing the efforts of the Manhattan Project.
• 1945 – August 15 – Japan surrenders to the Allied Powers.
• 1945 – August – The Soviet atomic bomb project is accelerated under a Special Commission chaired by Lavrentiy Beria. The program would be heavily reliant on espionage on the Manhattan Project, especially by Fuchs and Theodore A. Hall.
• 1945 – October 18 – The Atomic Energy Commission is established in France by French President Charles de Gaulle to investigate military uses of atomic energy.
• 1946 – January – The Atomic Energy Act of 1946 takes effect, officially turning over the Manhattan Project to the United States Atomic Energy Commission.
• 1946 – March 26 – The Strategic Air Command is established in the U.S. Army Air Forces for command and control of nuclear weapons.
• 1946 – April – Conference is held at Los Alamos that concludes that a "Super" bomb can likely be built.
• 1946 – June – First meeting of the United Nations Atomic Energy Commission, which was established by the first resolution of the U.N. General Assembly, is held.
• 1946 – June – The Soviet Union rejects the Baruch Plan.
• 1946 – August – The Convair B-36 Peacemaker is introduced as the first purpose-built nuclear bomber.
• 1946 – December 25 – The Soviet Union activates the F-1 pile in Moscow, producing the first controlled nuclear reaction in Europe.
• 1947 – The RTV-A-2 Hiroc, the first design of an intercontinental ballistic missile, is cancelled by the United States.
• 1947 – A steppe near Semipalatinsk, Kazakh SSR is selected by Beria as the Soviet Union's nuclear test site.
• 1947 – January – British prime minister Clement Attlee approves the development of an atomic bomb through the High Explosive Research programme led by William Penney, Baron Penney.
• 1948 – June 19 – The Soviet Union's first plutonium production reactor is activated at Chelyabinsk-40.
• 1948 – Andrei Sakharov proposes the first design for a Soviet hydrogen bomb.
• 1948 – Indian Prime Minister Jawaharlal Nehru ratifies an act establishing the Atomic Energy Commission of India chaired by Homi J. Bhabha.
• 1948 – September – The Soviet Union launches its first ballistic missile, a reverse-engineered version of the V-2 rocket later renamed the R-1 rocket.
• 1948 – The United States transfers nuclear-capable B-29 bombers to Europe during the Berlin Blockade.
• 1949 – August 29 – The Soviet Union conducts its first atomic test, RDS-1.
• 1949 – September 3 – U.S. atmospheric monitoring flights begin detecting effects of the Soviet test.
• 1949 – September 23 – President Truman announces that the Soviets have conducted an atomic test.
• 1949 – September through December – Debate occurs within the Truman administration over whether to authorize the development of a hydrogen bomb. Although the AEC General Advisory Committee chaired by Oppenheimer condemns the idea, the bomb is encouraged by the Department of State, the Department of Defense, the Joint Chiefs of Staff, the Joint Committee on Atomic Energy, and the National Security Council.
• 1949 – The U.S. Department of Defense prepares Operation Dropshot, a contingency plan for a nuclear and conventional war against the Soviet Union.
• 1949 – Following the Berlin Blockade and the articulation of the Truman Doctrine, the North Atlantic Treaty is ratified by 22 signatories in Western Europe and North America, including the United States, creating the collective security alliance NATO. The Treaty places its members under an American "nuclear umbrella" against a Soviet attack and provides the basis for nuclear weapons sharing agreements with Italy, the Netherlands, and Belgium.