atomic weapons. US military planners thought that the Soviet Union was too backward to develop a bomb before 1970; when US spy planes found radioactive isotopes over Soviet airspace, it touched off a search for communist spies inside of American political and military institutions. In order to appear strong against communists, American politicians put more emphasis on defense spending and containment as well as oppressing communist sympathizers at home.

The United States also had additional worries about the spread of communism after Mao's takeover of China. Even though the regime of Chiang Kai-shek was ineffective and corrupt, people were shocked when Mao's revolution took hold in China. There was additional shock when communists in North Korea invaded South Korea. Harry Truman was accused of being soft on communism as the press, led by Time's Henry Luce, stated that the United States "lost China." In order to ensure no further losses, presidents Eisenhower and Kennedy toughened their stance against anything that looked like communist aggression. It was believed that all communists were being directed by the Kremlin, when in reality, many of these communist takeovers were really anti-imperialist movements that used funding from China and the Soviet Union.

Communism was the official ideology of the Soviet Union. As the USSR and the United States were locked in a bitter rivalry throughout the Cold War, it's not surprising that successive administrations tried to contain the spread of its governing ideology. Communism in both theory and practice was implacably hostile to the United States and its institutions. The USA was the world's largest economy, a capitalist economy no less, and therefore epitomized everything that Communism was fighting against. The further this ideology spread throughout the world, the more hostile the international environment would become for American strategic and economic interests.

As both the United States and the Soviet Union were nuclear powers, direct confrontation was unthinkable. So successive American governments adopted the strategy of containment to deal with the Communist threat. In practice, this meant that the United States would not actively try to wrest back Eastern Europe from Soviet dominance and control as this could well lead to nuclear conflict. Instead, the spread of Communism beyond the Soviet sphere of influence would be held back by a number of other, more sophisticated measures. These included the provision of substantial economic and military aid to Western countries at serious risk of Communist subversion, such as France and Italy. In the developing world, the United States also backed a variety of dictatorships and resistance movements to crush the merest hint of Communist insurrection. American involvement in South Vietnam is a paradigm example of this approach.

There are at least two ways to answer this question. 

First, we can look at why the United States thought communism was an evil influence in the world.  The basic reason for this is that communism is a system in which people’s liberties count for little or nothing.  Communism as a system inherently believes in depriving people of economic liberty.  The government controls the economy and people are not free own much property or to do as they wish with what property they are allowed to own.  Communism as practiced also had little respect for human rights.  There was no right to freedom of religion and certainly no right to freedom of speech or the press, for example.  For these reasons, Americans saw communism as a menace to freedom.

Second, we can look at why Americans thought that communism might expand and would need to be contained.  This is largely because the Soviet Union grabbed for all of Eastern Europe after WWII.  Stalin wanted buffer states between the USSR and the West and therefore took control of all of Eastern Europe. When combined with the fact that communist ideology calls for communism to take over the world, these actions on Stalin’s part seemed like proof that communism was going to try to spread.  Therefore, it needed to be contained.