Aug. 13, 1942 - President Franklin D. Roosevelt signs order creating the Manhattan District in the U.S. Army Corps of Engineers.
Dec. 2, 1942 - Scientists at the University of Chicago achieve the first man-made nuclear chain reaction.
Dec. 7, 1942 - Secretary of War Henry Stimson notified the Los Alamos Ranch School that the government was taking over the property. Condemnation proceedings were begun for all property in the town and the papers were sealed. They weren't opened until 1961.
March 1943 - Scientists began arriving at Los Alamos to begin work on the Manhattan Project.
July 16, 1945 - The first atomic bomb was tested at the Trinity site on Alamagordo Air Force Base in New Mexico.
Aug. 6, 1945 - An atomic bomb explodes over Hiroshima, Japan, killing 78,000 people.
Aug. 9, 1945 - The second atomic bomb is dropped on Nagasaki, killing 25,000.
Aug. 15, 1945 - Emperor Hirohito signals that Japan is prepared to meet allied demands for unconditional surrender.
Sep. 23, 1949 - President Harry Truman announces the Soviet Union has tested an atomic bomb.
Oct. 3, 1952 - Great Britain tests an atomic bomb.
Nov. 1, 1952 - At Eniwetok Atoll in the Pacific Ocean, the United States tests a thermonuclear device, or hydrogen bomb, with the force of several million tons of TNT.
August 1953 - The Soviet Union detonates its first hydrogen bomb.
Oct. 4, 1957 - The space age begins with the Soviet launch of Sputnik.
Nov. 4, 1958 - The United States, Britain and the Soviet Union enter into a moratorium on testing.
Sep. 1, 1961 - The Soviets break the moratorium with a series of tests.
July 25, 1963 - The United States and the Soviet Union sign a treaty banning atmospheric nuclear tests.
March 5, 1970 - The Nuclear Nonproliferation Treaty goes into effect.
Dec. 27, 1985 - North Korea announces it will sign the nonproliferation treaty.
July 12, 1991 - Members of the U.N. Security Council warn Iraq to disclose its nuclear program.
Feb. 24, 1992 - The United States tells North Korea to open its nuclear program to international inspection.
March 11, 1993 - North Korea says it plans to withdraw from the nonproliferation treaty.
March 24, 1993 - President F. W. de Klerk admits that South Africa built six nuclear bombs. He said he ordered them dismantled and the program abandoned.
July 3, 1993 - President Clinton announces a 15-month extension of the moratorium on U.S. nuclear testing.
Oct. 5, 1993 - Defying an international moratorium, China conducts an underground nuclear test.
June 22, 1994 - North Korea agrees to freeze its nuclear program. The United States says it will reopen talks with the communist regime.
Aug. 10, 1994 - Three men are arrested at Frankfurt, Germany, airport after a flight from Moscow. In their luggage is a little over a pound of weapons grade plutonium.