In the 1950s, Russian and American scientists knew there was no limit to the power of thermonuclear hydrogen bombs.

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Meanwhile, both Russia and the United States had developed hydrogen bombs.

WHITEHOUSE: Dwight D. Eisenhower

Union would acquire atomic and then hydrogen bombs.

CENTERFORSECURITYPOLICY: Freedom Flame 2013

If, at the time of the Cuban missile crisis, for example, which you and I remember because we were around, it had all gone wrong and the Russians had put a bolt from the blue on London and Mr MacMillan was killed, the decision whether to launch the B-force, the B-bombers with their hydrogen bombs, would have gone to the Chief of Bomber Command, an airman in uniform.

BBC

After the Soviet Union broke America's atomic monopoly in 1949, several prominent scientists and politicians favoured the development of the hydrogen bomb, a fusion weapon hundreds of times more powerful than the bombs that the Manhattan Project had built.

ECONOMIST: Robert Oppenheimer