From the perspective of the United States, the U.S.S.R. reneged on its pledge to hold free elections in occupied territories in Eastern Europe after World War II. They also developed an atomic bomb and worked to advance communism around the globe, such as in former European colonies in Africa. Despite tensions, the Soviets also supported Mao's government in China and supported communist movements in Asia.
The United States was alarmed by Soviet global ambitions and engaged in a "cold" war in which the two superpowers did everything short of a "hot" war to contain each other. A "hot" war would have meant Soviet and U.S. armies meeting in battle, and very probably, the use of nuclear weapons.
U.S. foreign policy focused on containment of the spread of communism. Domestically, this led to a fear of communists in our midst working to overthrow our government. While the fears were exaggerated, they gave political power to politicians such as Joseph McCarthy, who used fear of communism to investigate and often destroy the careers of actors, artists, scholars and others who were accused, usually unfairly, of being a threat. Because of the fear of being accused of communism or of being called out as unAmerican, many people were careful to conform to societal norms during the 1950s. However, these were usually people with something at stake: in the mostly conformist 1950s a counter-culture of beatniks existed largely under the radar and would emerge more prominently in the 1960s as hippies.