The second British plutonium reactor starts operation in Windscale, Cumberland to manufacture plutonium for nuclear weapons.

U.S. mathematician Stanislaw Ulam proposes radically new design for the hydrogen bomb. Edward Teller embraces and refines Ulam's concept.

President Harry Truman approves the establishment of the Nevada Proving Grounds, later called the "Nevada Test Site" (NTS).

The first nuclear test (shot "Able") occurs at the Nevada Test Site.

Julius and Ethel Rosenberg are convicted at the Federal Courthouse in Foley Square, New York City and sentenced to death for passing information about atomic weapons to the USSR.

Edward Teller submits a report on the new design for the hydrogen bomb.

The U.S. Joint Chiefs of Staff order atomic retaliation against air bases in case of "a major attack" against UN forces in Korea.

President Harry Truman approves the military request to use atomic weapons in Manchuria if large numbers of Chinese troops join the Korean War, or if bombers are launched against United Nations forces from Manchurian bases. 

President Harry Truman discharges General Douglas MacArthur for insubordination after MacArthur repeatedly criticizes the limited objectives of the war in Korea.

During a top-secret, three-day conference at Los Alamos, New Mexico, scientists examine the feasibility of developing the hydrogen bomb.

U.S. nuclear test shot "George" (Operation Greenhouse) confirms for the first time that a fission device can produce the conditions needed to ignite a thermonuclear reaction.

Great Britain's first nuclear reactor goes online.

U.S. physicist Marshall Holloway is named leader of the hydrogen bomb project. Edward Teller leaves Los Alamos, New Mexico shortly afterward.

The Soviet Union conducts its second nuclear test: an improved plutonium bomb.

A four-man team at RAND begins to study the likely effects of the hydrogen bomb.