Which late 1940s event prompted the united states to take steps to protect itself from soviet espionage?

There were several events that led to the U.S. protection from espionage.  The detonation of an atomic bomb in Russia, defeat of the Nationalist by Communists in China, the Korean War and the conviction of Alger Hiss led the U.S. to protect itself from espionage.

photos creat an ovveral image of the civil war