Why did the us become involved on the Vietnam war?
Vietnam was part of the cold war. A war against communism.
Th US became involved in Vietnam in order to contain communism. US leaders believed a domino effect would cause other Southeast Asian to turn into communist countries. If the US never became involved, the horrible deaths on both sides of the war would have been avoided. It's clear that North Vietnam would take over South Vietnam. What is still unknown is whether other Southeast Asian countries would have become communist states.
The cold war caused the Vietnam War. The Atom Bomb created the cold war. If there had been no atomic bombs, there would have been no cold war. If there had been no cold war, the US may not have been involved in Vietnam in the first place. If the US had been involved, the US would have declared (total) war and defeated North Vietnam just like the US (allies) defeated Japan/Germany in WW2.