The correct answer would be "to stop the spread of communism." After World War II, the United States and Soviet Union emerged as global superpowers. These two countries had two very different political and economic philosophies, as the US supported capitalism and democracy while the Soviet Union supported communism. Considering this rival, it is easy to understand why the US would want to stop the spread of communism, as they opposed this system entirely.
This lead the US to get involved in the Vietnam War, as communism was threatening to take over South Vietnam. This system was used in North Vietnam and the goal of their leader was to unite Vietnam under one communist flag. The US wanted to ensure that this didn't happen, so they helped the military of South Vietnam.
