Timeline

919
June - Rutherford creates oxygen from nitrogen.

1920
Rutherford speculates on the existence of the neutron at the Royal Society.

1932
February - Chadwick discovers the neutron.

1933
October - Szilard recollects that "It occurred to me in October, 1933 that a chain reaction might be set up if an element could be found that would emit two neutrons when it swallowed one neutron." This idea became a classified British patent in 1935 before the fission was discovered.

1934
Artificial radioactivity discovered by Curie/Joliot (bombardment with alpha particles) and by Fermi (bombardment with neutrons).

1938
December 22 - Hahn and Strassmann (later Meiter and Frisch) conclude that the identification of barium implies that the uranium nucleus has been fissioned by neutrons.

1939
January to May - Many experiments on uranium fission.
August 2 - Szilard, Wigner, and Teller obtain a letter from Einstein on the possibility of a uranium weapon; Roosevelt receives the letter on October 11, 1939 from Sachs.

1940
June 3 - German scientists fail to observe neutron multiplication in the reactor in Hamburg.

1941
January - Based on experiments with a natural uranium reactor, the Germans reject graphite as a moderator.
July - British 'Maud' Committee reports that a weapon could be made with 10 kg of Uranium-235; U.S. Academy of Sciences endorses bomb program.

1942
May - Heisenberg and Dopel observe the first multiplication of neutrons.
December 2 - First nuclear chain reaction at Chicago's Stagg Field by Fermi.

1943
March 15 - Oppenheimer moves the bomb development to Los Alamos.

1944
August 26 - Bohr presents his memorandum on intentional control of nuclear weapons to Roosevelt.

1945
January 20 - First Uranium-235 separated at Oak Ridge.
July 16 - U.S. explodes first atomic bomb, the Trinity test, at Alamogordo.
August 6,9 - Atomic bombs dropped on Hiroshima and Nagasaki.

1946
June 14 - Baruch presents the Acheson-Lilienthal plan to internationalize the atom to the U.N.
June 30 - First subsurface detonation by U.S. at Bikini atoll.

1948
April, May - U.S. atomic tests, Eniwetok Atoll.

1949
August 29 - First Soviet detonation, in the Ustyurt desert.
October 30 - General Advisory Committee of the AEC recommends that the more powerful atomic bombs should be built rather than hydrogen bombs.

1950
January 27 - Fuchs confesses that he gave atomic secrets to the Soviets.
U.S. above-ground tests start in Nevada.
January 31 - Truman announces the decision to proceed with the H-bomb.

1952
October 3 - First British atomic detonation, Monte Bello Islands, Australia.
October 31 - U.S. explodes first fusion device, Mike, of 10.4 Mt at Eniwetok.

1953
August 12 - First Soviet fusion device exploded on a tower in Siberia.

1954
March 1 - The 'BRAVO' H-bomb test; Marshall Islanders and the crew of the "Lucky Dragon"are affected by fallout.
April 12 to May 6 - Oppenheimer hearings deny him access to nuclear secrets.

1955
November 22 - First fusion device test by the Soviet Union, it had a yield of 1.6 megatons. The development was lead by Andrei Sakharov.

1957
May 15 - First British H-bomb exploded at Christmas Island.
September 19 - First underground test, Rainier, 1.7 kilotons

1958
November 1958 to September 1961 - U.S., U.K., and U.S.S.R. observe an informal moratorium on nuclear tests.

1960
February 13 - First nuclear test by France, Sahara desert.

1961
September 1 - U.S.S.R. resumes nuclear tests.
September 15 - U.S. resumes nuclear test
October 31 - U.S.S.R. explodes the largest nuclear bomb, with a yield of a 58 megatons.

1963
June 20 - U.S./U.S.S.R. sign 'hot line' agreement.
August 5 - Limited Test Ban Treaty signed.

1964
October 16 - China (PRC) explodes first nuclear bomb.

1966
September 24 - First French H-bomb, Tuamoto Islands.

1967
January 27 - Outer Space Treaty bans nuclear weapons being placed in orbit.
February 14 - Treaty of Tlatelolco signed.
June 17 - First fusion device test by the Chinese, with a yield of 3 megatons.

1968
July 1 - Non-Proliferation Treaty opened for additional signatures.

1969
November to December - Preliminary SALT talks in Helsinki.

1972
May 26 - SALT I Treaty signed by Nixon and Brezhnev in Moscow.
November - SALT II negotiations begin.

1974
May 18 - India sets off a low-yield device (10-15 kt) under Rajasthan desert.

1978
April - The United States cancels development of the neutron bomb.

1979
June 18 - SALT II Treaty is signed in Vienna by Brezhnev and Carter.
December 26 - U.S.S.R. invades Afghanistan; SALT II Treaty removed from consideration by the Senate.

1982
June 29 - Strategic Arms Reduction Talks (START) begin in Geneva.

1985
The South Pacific Nuclear Free Zone Treaty - Multilateral agreement among the nations of the South Pacific.
August - The Soviet Union announces a nuclear testing moratorium.

1986
November 28 - The 131st U.S. B-52 bomber with ALCMs is deployed, exceeding the 130 limit, thus negating the SALT II Treaty.

1987
December 8 - Reagan and Gorbachev sign the INF Treaty.

1991
The START treaty is signed, signifiying further reductions in nuclear arms.

1992
The START treaty is ratified by the Senate.Kazakhstan, Ukraine, and Belarus agree in principle to the START treaty.

1994
Several attempts to smuggle nuclear material from C.I.S. are stopped.
Secret nuclear testing on humans is revealed by both the U.S. and C.I.S.
North Korea is suspected to be building nuclear weapons, they threaten to withdraw from the IAEA. Global pressure resolves the crisis.

1995
May 11 - 178 nations renew the Non-proliferation Treaty.
August - The U.S. annouces a total ban on all U.S. nuclear weapon testing.
September 5 - Under worldwide protest, France resumes nuclear testing in the South Pacific.

1996
March 25 - The United States, France, and United Kingdom sign The Treaty of Rarotonga, which created a South Pacific Nuclear Free Zone. China and C.I.S had already signed the treaty.
April 11 - The Treaty of Pelindaba creates a Nuclear Free Zone throughout Africa. Multilateral agreement signed by 49 of the 53 members of the Organization of African Unity
November 26 - The last of the nuclear warheads stationed in Belarus were finally removed to Russia. This completes the planned denuclearization of the smaller successor states to the Soviet Union, leaving Russia as the sole inheritor of the Soviet Union's nuclear arsenal.