What was the United Center Central motivation of fighting the Vietnam war

The United States' central motivation was to prevent the spread of communism throughout the region. They feared that if one country became fully communist, all others would soon follow, which was called the domino effect. That's why they started meddling in that are. First in Korea which was a successful front, and then in Vietnam which wasn't successful and the country became communist eventually.

For oil or because America said a small patrol boat shot at them