German scientists split the atom.
September — World War II starts when Germany invades Poland.
September — German scientists learn that uranium research has started in England and the United States.
October — Alexander Sachs, a friend of both President Franklin D. Roosevelt and Albert Einstein, delivers a letter from Einstein to Roosevelt explaining that a chain reaction could lead to the construction of an extremely powerful bomb.
February — In Minneapolis, scientist Al Nier first separates a small amount of uranium-235 necessary to build the uranium bomb.
March — Scientist John Dunning, working with Niels Bohr, shows fission is more easily produced in the rare uranium-235 than in the more plentiful uranium-238.
April — Japanese studies into atomic energy begin with the military.
November — Roosevelt is elected to third term as U.S. president
February — Scientists in Berkeley, Calif., produce a new element, eventually to be named plutonium.
June — Germany invades the Soviet Union.
September — A committee of elite British scientists determines an atomic bomb is possible and should be built.
December — President Roosevelt appoints a committee to determine if the United States should build the bomb and what it will cost.
Dec. 7 — Japan attacks Pearl Harbor.
January — Roosevelt urges scientists to develop an atomic bomb.
May — U.S. scientists decide to move ahead on five different methods to produce fissionable material.
May — German armament minister Albert Speer discusses an atomic weapon with Hitler.
September — Gen. Leslie Groves assumes control of the Manhattan Project. He selects Oak Ridge, Tenn., as the site for the uranium production plant.
October — Groves meets with J. Robert Oppenheimer to discuss the physicist’s interest in managing the Manhattan Project. A week later, Oppenheimer’s acceptance is final.
December — U.S. scientists led by Enrico Fermi working at the University of Chicago achieve critical mass for the first time in history.
December — U.S. government takes over the Los Alamos Ranch School for military purposes. The ranch, founded in 1917, and 54,000 acres around it is designated Site Y.
January — Groves selects Hanford, Wash., as the site for plutonium production reactors.
March — Oppenheimer arrives in Santa Fe.
April — The new laboratory at Los Alamos officially opens. The technical areas had no power, telephones or equipment.
April — Nearly 100 scientists are in Los Alamos and ready to start work. Their average age is 25.
April — A cyclotron from Harvard University arrives at Los Alamos and is one of the first experimental devices at the site.
June — First implosion experiments take place.
July — First shipments of plutonium arrive.
August — Roosevelt and British leader Winston Churchill sign agreement pledging collaboration on the bomb.
October — Danish scientist Niels Bohr escapes from Sweden and comes to Los Alamos in December.
February — Physician Louis Hempelmann reports on hazards of exposure to radiation.
June — Oppenheimer announces the existence of spontaneous fission in plutonium-240, making possible a gun-assembled weapon using plutonium.
July — Explosions are heard in the canyons around Los Alamos as scientists test the theory of implosion.
August — The “Gadget Division” is organized at the laboratory to centralize work on weapons physics.
August — British scientist and Soviet agent Klaus Fuchs arrives in Los Alamos as does Soviet spy David Greenglass, the brother of another Soviet recruit, Ethel Rosenberg.
September — Enrico Fermi becomes associate director of the laboratory.
September — Gen. Groves makes arrangements to acquire the northwest corner of a bombing range not far from Socorro. Oppenheimer chooses the code name Trinity.
December — Construction at Trinity is completed.
February 2 — First large shipment of plutonium reaches Los Alamos.
March 9 — B-29 raid on Tokyo kills 83,793 people and destroys more than a million buildings.
April 12 — President Roosevelt dies. Harry Truman is sworn in and told about the atomic bomb.
April 27 — GIs at Los Alamos and Trinity hear that all furloughs are canceled until after July.
May 7 — German army surrenders.
May 25 — Joint Chiefs of Staff sets November as date for invasion of Japan.
May 31 — At a congressional meeting to discuss the use of the bomb, Oppenheimer estimates 20,000 people will die in an atomic bomb explosion.
June 2 — Fuchs meets his contact Harry Gold in Santa Fe and reveals scientists at Los Alamos are making tremendous progress and plan a test in July.
June 12 — At the bottom of Los Alamos Canyon, scientist Louis Slotin assembles, for the first time, two full-scale plutonium hemispheres in a test for criticality.
June 16 — The Interim Committee scientific panel says it sees no alternative to direct military use of the bomb.
June 19 — In Flushing, N.Y., Harry Gold meets with his Soviet superior and turns over information about the implosion lens and other matters given to him by Fuchs and Greenglass.
June 24 — Scientists establish the size of the Trinity bomb.
July 3 — Leo Szilard circulates a petition intended for Truman, asking that atomic bombs not be used.
July 6 — Truman leaves Washington for the Potsdam Conference near Berlin.
July 13 — “The Gadget” is assembled at Trinity Site.
July 14 — The Gadget is hoisted atop 100-foot tower.
July 16 — The first atomic bomb is exploded at precisely 5:29:45, sending a fireball eight miles into the sky. After watching his handiwork, Oppenheimer recalls the Hindu epic Bhagavad Gita and a quotation about Lord Krishna: “I am become death, destroyer of worlds.” Another scientist says: “Oppie, now we’re all sons of bitches.”