Why did the United States enter the Vietnam War?

Why this is the answer

Vietnam has the same single answer as Korea: the United States entered the war to stop the spread of communism. Vietnam was divided. A communist government held the north, and the United States backed the government in the south, helping it resist pressure from the communist north. American leaders saw the war through the domino theory, the Cold War belief that if one country fell to communism its neighbors would follow. The same reasoning, first laid out with the Truman Doctrine, had already been used to justify action in Korea. American involvement grew in stages. U.S. forces fought alongside the South Vietnamese Army from 1959, but the big escalation followed an incident in August 1964, when North Vietnamese torpedo boats attacked the destroyer USS Maddox in the Gulf of Tonkin. On August 7, Congress passed the Tonkin Gulf Resolution, which approved "all necessary measures" to repel attacks on American forces. Only two senators voted against it. Congress never declared war, and that resolution became the legal basis for a long, undeclared war. It turned controversial later: a second attack reported two days after the first was eventually found never to have happened, and Congress repealed the resolution in January 1971. The war ended in 1975, when Saigon, the capital of South Vietnam, fell. By 1976 the whole country was under communist control. Almost 60,000 American servicemembers died or were listed as missing. In all, American forces fought in Vietnam for about sixteen years. Because the answers for Korea and Vietnam match word for word, it pays to learn them as a pair and in order: Korea first, then Vietnam, both in Asia, both against the spread of communism. It is tempting to answer with the Gulf of Tonkin, since that is when the fighting grew. That incident was the trigger, not the reason, and it is not the answer on the list. Keep to the official words.