Why did the u. s. begin fighting the vietnam war in 1964? was the fighting justified?

The US believed that if Vietnam fell to communism, all other SEA countries would follow. This was described as the domino effect, so US intervened so as to prevent the spread of communism. Was it justified is a hard question to answer since the people were completely split when it came to that question. Many believed that the war effort was wrong and that US should retreat, while many also believed that it was a war for freedom and democracy.