By December 24, 1945, the U.S. Embassy in Moscow warned of a determined and concentrated effort by the USSR to build an atomic bomb. The Soviets developed their own atomic and hydrogen bombs shortly after the US.
On February 9, 1946, Joseph Stalin gave a speech at the Bolshoi Theatre, marking the deterioration in relations with the U.S. Two months later, Soviet scientist Iulii Khariton chose Sarov/Arzamas as the confidential location for Soviet weapons lab. .
On November 10, 1946 there was the first full-scale assembly of a nuclear reactor, by a team of Soviet scientists headed by Igor Kurchatov. On Christmas day Soviet scientists achieved nuclear chain reaction, one of the primary steps in building the atomic bomb. .
In October 1947, Joint Chiefs of Staff declare that 150 "Nagasaki type" bombs would be sufficient to defend the United States and defeat the USSR. The hoard of weapons was still undersized at that point (20-50) but growing. .
In April of 1948 the Atomic Energy Commission started "Operation Sandstone" at Eniwetok Atoll in the Pacific Ocean to test the improved designs of nuclear fission bombs (nuclear fission is used in atomic bombs; nuclear fusion is used in hydrogen bombs). Later, in July, Andrei Sakharov, a Soviet physicist, begins the development of the "Layer Cake" concept for the hydrogen bomb. .
In late August of 1949 the first Soviet atomic bomb test is exploded in Kazakhstan. This shows the progress of the Soviets in their nuclear technology, comparable to the Americans, whom are more advanced at this moment. .
On January 31st of the next year, 1950, President Truman states his decision to start developing the hydrogen bomb. Five days later, in February, twelve leading American physicists speak out against President Truman's decision to build the H-bomb. However, the Joint Intelligence Committee foretold the build up of the Soviet atomic arsenal and the chance of an attack against the United States "at earliest possible moment".
