Regarding the policy of containment, was the United States correct to think of the Soviet Union as expansionist and as a threat to American liberty?

As the Second World War progressed into its later years, the United States and Soviet Union, once allies, became bitter rivals with their vastly different political and economic philosophies: democracy/capitalism (US) and communism (Soviet Union).  Containment was considered to be a remedial policy by the United States as a way to stop, or "contain," the spread of Communism under the Soviet Union's influence.  The belief of the US was that if the conditions were right and the Soviet Union was able to transition its neighbors to convert to its political philosophy, widespread Communism would potentially sweep the world and seriously impact the influence of Democratic adversaries, such as the US.  

The idea of Containment was spread by George F. Kennan, a diplomat who had worked in the US embassy in the Soviet Union for 20 years.  The idea was that the United States would not try to impact already Communist countries directly but rather would contain the spread of it and would aid US allies in their actions to impede the spread of Communism.

In terms of the Soviet Union being expansionist, evidence suggests that by 1948, seven East European countries were controlled by Communist governments.  As Churchill indicated, the Soviet Union maintained control from behind the "iron curtain."  When we think about evidence regarding the success of the policy of containment, we must consider that the United States and its allies were successful in stopping the spread of Soviet influence in both Turkey and Greece.  Based on this, it is safe to assume that the Soviet Union was certainly attempting to spread its influence, and the United States was successful in its actions to stop the spread of Communism.  Ultimately, we must consider the number of Communist countries over time has dwindled significantly while the number of democratic countries continues to climb.