Why did the United States enter the Vietnam War?

What does this answer mean?

The United States entered the Vietnam War to stop the spread of communism. That's the answer USCIS accepts, and it's the same as the answer for the Korean War.

During the Cold War, the United States followed a policy of containment, trying to keep communism from spreading to more countries. In Vietnam, the country was divided between communist North Vietnam and South Vietnam, and the United States supported South Vietnam. American involvement grew over many years.

In January 1973, the United States, North Vietnam, South Vietnam, and the Provisional Revolutionary Government signed the Paris Peace Accords. The agreement let the United States leave the fighting and bring home its prisoners of war, and the last American troops left Vietnam on March 29, 1973. Fighting between the Vietnamese sides continued after that.

At the interview, say "to stop the spread of communism."