How did the United States react when the Soviet Union exploded its first atomic bomb in 1949?
The Soviet Union's successful detonation of the atomic bomb in 1949 contributed to the already tense domestic political atmosphere in the United States, and, it could be argued, played a role in ratcheting the tension up to the point of hysteria. The House Un-American Activities Committee intensified its activities, particularly with respect to Communism in Hollywood, and the knowledge that there had been spies within the US atomic program made things even worse.
In short, the bomb caused fears that the United States was losing the Cold War, and that insidious forces in the United States were undermining the struggle against communism. The judge who sentenced Julius and Ethel Rosenberg to death for espionage said as much, specifically blaming the ongoing conflict on the Korea peninsula on the fact that the Soviets had the bomb:
Putting into the hands of the Russians the A-bomb has already caused, in my opinion, the Communist aggression in Korea, with the resultant casualties exceeding 50,000 and who knows but what that millions more innocent people may pay the price of your treason.
In the heated atmosphere surrounding the trial, the fact that the Rosenbergs had not been either accused or convicted of treason seemed not to matter. This case was an example of the visceral, almost existential angst that characterized America's response to the Soviet bomb test.