In the immediate aftermath of World War II , the leadership of the Soviet Union feared the United States would use its nuclear superiority to its advantage , as from 1945 to 1948 the U.S. was the only state possessing nuclear weapons . The USSR countered by rapidly developing their own nuclear weapons , surprising the US with their first test in 1949 . In turn , the U.S. countered by developing the vastly more powerful thermonuclear weapon , testing their first hydrogen bomb in 1952 at Ivy Mike , but the USSR quickly countered by testing their own thermonuclear weapons , with a test in 1953 of a semi-thermonuclear weapon of the Sloika design , and in 1956 , with the testing of Sakharov 's Third Idea – equivalent to the Castle Bravo device . Meanwhile , tensions between the two nations rose as 1956 saw the suppression of Hungary by the Soviets ; the U.S. and European nations drew certain conclusions from that event , while in the U.S. , a powerful social backlash was afoot , prompted by Senator Joseph McCarthy , the House Un-American Activities Committee , and Julius and Ethel Rosenberg , U.S. citizens executed in 1953 after conviction of espionage . This atmosphere was further inflamed by the 1957 launch of Sputnik , which led to fears of Communists attacking from outer space , as well as concerns that if the Soviets could launch a device into orbit , they could equally cause a device to re-enter the atmosphere and impact any part of the planet . John F. Kennedy capitalized on this situation by emphasizing the Bomber gap and the Missile gap , areas in which the Soviets were ( inaccurately ) perceived as leading the United States , while heated Soviet rhetoric added to political pressure . The 1960 U-2 incident , involving Francis Gary Powers , as well as the Berlin Crisis , along with the test of the Tsar Bomba , escalated tensions still further
Castle Bravo
Castle Bravo
Pre-emptive nuclear strike