 The perception of communism in the US was shaped by the Cold War , which began after World War II when the Soviet Union failed to uphold the commitments it made at the Yalta Conference . Instead of holding elections for new governments , as agreed at Yalta , the Soviet Union occupied several Eastern European countries , leading to a strained relationship with the US . Subsequent international events served to increase the apparent danger that communism posed to Americans : the Stalinist threats in the Greek Civil War ( 1946 – 1949 ) ; the Czechoslovak coup d 'état of 1948 ; and the 1948 blockade of Berlin . 
