USCIS 128:110. Why did the United States enter the Korean War?

OpenAI. (2026). Why did the United States enter the Korean War?  [AI-generated infographic]. OpenAI.  Edited by Jennifer Gagliardi

After World War II, Korea was divided into two separate countries along the 38th parallel. North Korea became a communist country supported by the Soviet Union and later by China. South Korea became a non-communist country supported by the United States and its allies.

At this time, the Cold War had begun. The United States and the Soviet Union were rivals with very different political systems. The United States supported democracy and capitalism, while the Soviet Union supported communism.

American leaders worried that communism would spread to other countries. Many Americans believed in the domino theory, the idea that if one country became communist, nearby countries might also become communist.

Because of these fears, the United States followed a policy called containment. The goal of containment was to stop the spread of communism around the world.

On June 25, 1950, North Korean forces invaded South Korea. North Korea tried to unite the Korean Peninsula under communist control.

President Harry S. Truman  believed the invasion was part of communist expansion during the Cold War. He decided that the United States should help defend South Korea.

The United Nations also voted to support South Korea. Soldiers from the United States and other United Nations countries fought to stop the North Korean invasion.

At first, United Nations forces pushed North Korean troops back toward the Chinese border. However, later in the war, Chinese forces entered Korea to support North Korea. The fighting became long, difficult, and deadly.

Many American soldiers served in Korea under the command of General Douglas MacArthur. These soldiers served in newly integrated military units created after Executive Order 9981.  This order, signed by President Truman in 1948, ended racial segregation in the U.S. military.

The integration of the military became an important step in the Civil Rights Movement. African American soldiers served with white, Latino, and Asian American soldiers in the same units during the war. 

After returning home, many African American veterans demanded equal rights and an end to segregation in American society. The success of integrated military units helped increase support for civil rights reforms during the 1950s and 1960s.

In 1952, Dwight D. Eisenhower was elected president. Eisenhower promised to help end the war. After negotiations continued, the fighting ended with an armistice agreement in 1953 during Eisenhower’s presidency.

However, no official peace treaty was signed. Korea remained divided into North Korea and South Korea near the 38th parallel.

The Korean War is important because it was one of the first major conflicts of the Cold War. The United States entered the war mainly to stop the spread of communism in Asia. The war also took place during an important period of change in the U.S. military and the growing Civil Rights Movement in the United States.