What did the Soviets do for the first time in 1949?

The successful test of RDS-1 in August of 1949 inspired the Soviet government to institute a major, high-priority program to develop the hydrogen bomb. The Soviets, who received information from Klaus Fuchs regarding the American hydrogen bomb program throughout the late 1940s, knew that thermonuclear weapons were theoretically possible. They also knew that the hydrogen bomb would have to be developed in order to counter the perceived "American threat" abroad.

The effort to develop a thermonuclear weapon was led by Soviet physicist Andrei Sakharov, widely considered the "Father of the Soviet H-Bomb." Sakharov was the author of several key ideas that contributed to the Soviets' thermonuclear design and his research played a key role in the development process.

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In 1949, the Soviets conducted their first successful nuclear weapons test, marking their entry into the nuclear arms race. This event occurred on August 29, 1949, at the Semipalatinsk Test Site in Kazakhstan. The test, codenamed "RDS-1" by the Soviets and dubbed "Joe-1" by the West, demonstrated their capability to produce nuclear weapons, thereby significantly altering the geopolitical landscape and escalating tensions between the Soviet Union and the Western powers, particularly the United States.