There are several, but historians argue the most terrifying event for the US was the Soviet atomic bomb detonation.
September 1949, the Soviet Union exploded its own atomic bomb and this shocked the U.S. because now both nations could destroy each other. -This was the world's first human-made satellite.
The second event was the cold war. The main reason why the United States was fearful of the Soviet Union in the "Cold War" was because the US was afraid that communism would spread around the world and ultimately to the US--which they viewed as being a threat to their way of life.
