In October of 1949, Mao Zedong and his Communist forces took over China and expelled the opposing Nationalist forces to the island of Taiwan. The Communist takeover of China was alarming to many Americans. In the same year, the Soviet Union detonated its first atomic bomb, and this event was also threatening to Americans, who were chagrined to see that their opponent in the Cold War also had nuclear capabilities. Shortly thereafter, Klaus Fuchs, a physicist who had been born in Germany and who later helped the US build its own atomic bomb, was accused (and later convicted) of giving nuclear secrets to the Soviets. In response, President Truman decided to pursue building the hydrogen bomb, which was even more deadly than the atomic bomb (see the source below).

In June of 1950, forces from North Korea, backed by the Soviet Union and China, invaded South Korea. Americans, under the aegis of the United Nations, were involved in fighting the Korean War until a stalemate ended the war in 1953. The Korean War was the first major military engagement of the Cold War.

There were a few events dealing with communism that alarmed the American people between 1949 and 1953. The United States and the Soviet Union had been involved a struggle over the spread of communism since World War II ended. We provided help to countries that were resisting the spread of communism. For the most part, we were successful in preventing communism from spreading in Western Europe and in West Berlin. However, in 1949, things changed.

The United States had been supporting the non-communists in the Chinese Civil War. While the Chinese put aside their differences during World War II, the communists and non-communists resumed fighting after World War II ended. In 1949, the Chinese communists won the Chinese Civil War, forcing the non-communists to flee to Taiwan. For the first time, a country that we were actively supporting lost to the communists. This alarmed our people. Many people believed that communism would now spread to other places in Asia.

In 1950, North Korea, which was supported by the Soviet Union, invaded South Korea. North Korea wasn’t provoked into invading South Korea. North Korea wanted to unite Korea into a communist country. The United States went to the United Nations for help in dealing with this invasion. The United Nations created a multinational fighting force, led by the United States, to help South Korea remain independent. When the Korean War ended in 1953, South Korea remained noncommunist.

These two events alarmed the American people between 1949-1953.

The Chinese Revolution in 1949 put the Communists in power in the most populous nation on Earth.  Prior to that time, China was loosely allied with the former Allied nations that had fought World War II, as China had opposed Imperial Japan in the 1930's.  Losing China changed the world's political balance.

At the conclusion of World War II, Korea was occupied by both the Soviet Union and the United States, but by 1950 the North, supported by the Soviet Union, had invaded the South, supported by the US and the United Nations.  This was the first actual "hot" conflict of the Cold War, which concluded in 1953.