One thing to consider is that the American Congress did not initially support Truman's anti-Communist agenda. After the war, many still saw the Soviet Union as an American ally--after all, the two nations had just won a world war together. Another user commented that the fact that Marxism clashed with the values that Americans held dear played the decisive role in the United States' opposition to the Soviet Union after the war. While it is true that this divergence in values did play a substantial role in the Cold War, remember that the Soviet Union extolled the same Marxist message prior to the war without drawing the same kind of fear from the United States internationally (although the United States did have a prominent First Red Scare domestically after the rise of Bolshevism). Indeed, the key to understanding the fear that Americans had of communism after the war derived from the fact that the same Soviet Union that seemed impotent on the international stage in the 1920s now looked like the preeminent global threat.

At the end of the Second World War, the Soviet Union had an empire forming in Eastern Europe, or at least that's what the soon-to-be-enormously-influential diplomat George Keenan warned in his "Long Telegram" sent from Moscow in early 1946. Keenan warned that the Soviet Union was not a normal world power, that its fundamental philosophy compelled it to continue to try to expand its empire across the globe. In this telegram, as well as his unclassified followup to the telegram, the widely published "Article X," Keenan argued that the United States had to either "contain" communism or else the Soviet Union's grip over Europe and Asia would widen.

Truman was an early advocate of Keenan's position, and he warned Congress in the wake of Britain's withdrawal from Greece and Turkey that if the Americans did not fund that anti-Communist fighters there, then the Communists would win emerging civil wars in those nations and extend the Soviet Union's power. But while Congress did fund that initiative, they hesisated when Truman called for further assistance for Western Europe through the creation of NATO and the development of the Marshall Plan (a proposal to spend billions of dollars in order to stimulate the European economy). Congress hesitated to make such a strong military commitment to Western Europe or to enact a bill that would cost over ten times more than the aid package to Greece. Then in February of 1948, the Communists instituted a coup (at least from the American perspective) in Czechoslovakia, turning the only parliamentary democracy in the Soviet sphere into another Soviet satellite governed by single-party rule. This tipped Congress into supporting both of Truman's initiatives.

The year 1949 brought even more fear into the American mind. The civil war raging in China since the Second World War ended with a Communist victory, seemingly bringing another world power under the control of Stalin and the Soviet Union. Just two months prior to that, the Soviet Union had detonated its first atomic bomb, ending a nuclear monopoly that American policymakers had hoped to maintain into the 1950s. This fear compelled the United States to involve itself in its first proxy war against the Soviet Union (or, again, so it seemed to Americans at the time) when it intervened--with UN permission, which is a story in itself--in the Korean War. The war proved a major setback from the United States, as it drew China into the war and could not win the stalemate that followed.

All of this may sound terrifying enough, but fear did not peak until the years 1957-1962. In late 1957, the Soviet Union launched Sputnik into space, proving that it had greater rocket technology that the United States and thus the ability to develop an arsenal of atomic weapons that it could deliver across continents to attack the United States without similar retaliation. This moment spurred the "Space Race," which ended in 1969 when the United States landed a man on the moon. But well before that could happen, and still in the atmosphere of fear of nuclear annihilation, American intelligence discovered in late 1962 that the Soviet Union had moved nuclear missiles to Cuba. The ensuing "Cuban Missile Crisis" brought the two superpowers to the brink of nuclear war as John Kennedy threatened to attack Cuba if the Soviet Union did not withdraw--a fact followed closely by the American media--until finally the two powers reached a deal that involved the removal of the missiles from Cuba.

Ironically, one of the fundamental fears driving the Cold War--that Communism represented a monolithic force controlled in the center by Moscow--proved to be untrue in the long run. While the Soviet Union did maintain a high level of influence over Eastern Europe, it did not control the Communists in Greece as the Americans believed, nor in China, in Korea, and in Vietnam. In each case as Communism spread, local nationalists embraced the philosophy because of its anti-imperialist idealism and because of the substantial support it bestowed on them from Moscow. By the end of the 1970s, the Sino-Soviet split had already been exploited by Richard Nixon, and two Communist nations, Cambodia and Vietnam, had entered into a war. By exaggerating the threat that monolithic communism imposed on the United States, American policymakers had helped to create the very bipolarity that they sought to undermine in order to win the Cold War.

Communism was a concern long before World War II, and not just in the United States. I'll attempt to provide a bit of context for what the situation was prior to the post-war period.

The Entente forces during and after World War I were involved in the Russian revolution, aiding the White forces against the Reds (even Australians! There's a book called Anzacs in Arkhangel with information about this). As the objective is the overthrow of the capitalist system, the redistribution of wealth and the formation of a new kind of state, Communist revolutionaries were the natural enemies of both the capitalist democratic states as well as the autocratic ones. If you were invested in the continuation of the status quo, a communist revolution was not desirable.

In the United States, the communist movement was not as large as it was in some European countries during the 1930's, but there were nevertheless both socialist and communist movements in the nation, and not insignificant ones. Around 1919-1922, the US government did take action against communist movements, deporting foreign born members of the Communist Party of America and driving the organisation underground, but this activity eventually subsided.

In Europe, there had been a failed communist revolution in 1919 in Germany, during the transitional period between the German Empire and the Weimar Republic. In Spain, the Spanish Civil War was fought by the Nationalist faction against what they claimed to be "the Red Hordes" of anarchists and communists. In France, the communist party received 15% of the vote, and formed part of the "Popular Front" ruling coalition. In Germany, in 1932, the KDP (Communist Party) received 16% of the vote while the SDP (Social Democrats) received 20% of the vote. This is all to say, communism was a political force that was very much on the radar in Europe.

In 1939, while Germany was stealing most of the limelight, the Soviets signed the Molotov-Ribbentrop pact and began cooperating with the Germans. While not public knowledge at the time, the secret protocols of this pact divided Eastern Europe up into mutual spheres of influence of the Soviet Union and German Reich. The USSR annexed Moldavia, invaded and annexed Eastern Poland (under the pretence of "protecting minorities"), annexed Lithuania, Latvia and Estonia, and kicked off the Winter War against Finland resulting in border modifications. What I am attempting to illustrate here is that the USSR was not just a passive victim during this period, it was a cause of concern to other states in Europe not only for purely theoretical reasons, but based on its actions.

In 1945, the war ended with the Soviet Union in control of Manchuria, North Korea, Poland, parts of Germany, Romania, Bulgaria, Hungary, and Czechoslovakia. It was not immediately clear, but within a few years there were communist governments in charge of all of these places. The USSR was supporting the Communist faction in the Chinese civil war (side note: during the Second Sino Japanese war, the Soviets had also supported the Nationalist in opposition to China, which is a bit of a historical quirk), had failed to come to an agreement on the reunification of Germany, had failed to come to an agreement on the reunification of Korea, and the stage was more or less set for the Cold War as we know it. Communism had expanded its reach very substantially by 1949.

I will refer you to an informative post from this thread via u/TheNewTroll on the topic of "Domino Theory" specifically - the idea that if the United States did not stop communism from taking hold in one country, it would continue spreading to its neighbours. Specifically, there is a document referenced (appears at the bottom of the scanned document, you can navigate to the next pages with the arrows) where a lot of the thinking about the Soviet Union and communism in general in the United States for the early cold war period is outlined.