Why did the United States enter the Korean War?

What does this answer mean?

The United States entered the Korean War to stop the spread of communism. That's the answer USCIS accepts.

On June 25, 1950, the army of communist North Korea crossed the 38th parallel, the line dividing Korea, and invaded South Korea. President Harry Truman organized international opposition through the United Nations Security Council, and on June 27, 1950, he committed the first U.S. forces to combat in South Korea.

The war was part of the Cold War. The United States followed a policy of containment, trying to keep communism from spreading to more countries, and Korea was one of the first places where that policy led to war.

After three years of fighting, representatives of the United Nations Command and North Korea's army agreed to an armistice in July 1953, during President Eisenhower's first year in office. The armistice stopped the fighting, but it wasn't a permanent peace treaty.

The next question, about the Vietnam War, has the same answer. At the interview, say "to stop the spread of communism."