The United States began viewing Russia with suspicion long before World War II and it only increased during and after the war. The Bolshevik revolution and subsequent peace with Germany under the treaty of Brest Litovsk began initial suspicions of communist agendas. This suspicion only increased with reports of communist party activities like Lenin's Stalin's 1st and 2nd 5 year plan which both displayed communist aggression and disregard for many human rights as tens of millions would be starved out under collectivisation, or outright killed during Stalin's great purge. Suspicion also increased when the Soviets signed the Molotov-Ribentrop treaty in 1939 securing "cooperation" between the two nations. This pact also saw the USSR move into Poland just as Germany had.
Further, tension was built during the Yalta and Potsdam conferences wherein Stalin vigorously argued for the ability to install communist friendly governments in the Balkan and Eastern European states that they had "liberated" during the war. Between that and their stance on securing parts of Poland and reshaping their borders, the US began to realize the subervsive, expansionistic, and aggressive nature of the Soviet Union. This sentiment was only strengthened when the USSR declared their intent to exert influence in Manchuria and desire to occupy post war Japan.
After the war, the fall of China to Communism was seen as an act coordinated by Moscow. When Lenin had taken over early on he had developed the commuter to establish a communist world order, and though it had long been abolished, the precedent it set stuck with the West as a main communist agendas.
Other points to consider, when the war ended, Greece endured a civil war between communist and non-communist factions. The UK could not afford to stay and help and let the US know that either they help or greece would fall to communism which led directly the implementation of the Truman Doctrine and the Marshall Plan all of which shaped the Cold War.
