The United States joined in on the Vietnam War because they were scared of the spread of communism.
The Vietnam War was originally fought by North Vietnam, who were communists, and Southern Vietnam. The United States' prime goal was to stop the spread of communism, as North Vietnam wanted worldwide communism.
The United States ended up losing the war. They lost numerous soldiers and their reputation plummeted. The war went on for around 20 years, but the United States and Vietnam agreed on a cease-fire that properly ended the war.