Why did the United States go to war with North Vietnam?

The primary reason the United States went to war with North Vietnam was to stop the spread of communism. This was part of the broader Cold War strategy, known as the Domino Theory, which suggested that if one country in a region fell to communism, neighboring countries would also fall, leading to a wider spread of communism. While the U.S. did have an interest in supporting South Vietnam against a communist takeover and indirectly opposing Chinese influence in the region, the main justification for U.S. involvement was to prevent the spread of communism in Southeast Asia.