very briefly, a civil war between the communist north and the south. The USA entered on the side of the south because it feared that if the south lost it would go communist and then other countries all round it would do the same (the 'domino theory'). The south, and the USA, lost, so Vietnam is reunified and still communist, though the south is notably freer; but other nations did not go communist.
In fact the roots go much deeper than that, as Wikipedia will tell you. It was all part of the cold war, west v east, which basically ended with the collapse of communism, though there are still some communist countries like Vietnam, Cuba and North Korea - and China.