The United States entered the Vietnam War gradually over a number of years. They essentially took the place of the French who controlled IndoChina after their defeat in 1954. The US began to send military advisors to the region in the 1950s and 1960s. Formal troop commitment began in the mid-1960s.
Yes, they were justified because they had to prevent communism from spreading, which was the pledge of the Truman Doctrine
