The Vietnam war was one of the biggest conflicts that permeated the “Cold War”. This particular conflict in a series of global conflicts over a substantial amount of time in the second half of the 20th century, was what defined the cold war. If we look at this conflict through the lens of the IR Theory, Realism. As we have seen realism in essence focuses on power and security especially in the international system. With these ideas in mind we can look at the Vietnam conflict in the eyes of a realist. The question is why would a country like the US invade a country that is roughly the size of Florida? The answer is: Power.
With the world in a bipolar state between the USSR and US, they used proxy states to further their cause. The US above all feared the spread of communism and had the belief that if countries in and around the USSR’s sphere of influence began to assimilate into Soviet’s ideology then slowly all countries would do the same, as they called it; the domino effect. This fear led the US to take the foreign policy of reacting anywhere communism began to fester. This can be seen as relative gains, in the way that they can not allow the other power in the world to gain any more power than they already have.
Realists believe that the international system is in a state of anarchy, not chaos, but no overarching sovereign to govern the rest of the countries in the system. This belief leads them to look out for themselves and only themselves. As such, with the US hiding behind the statement that they were trying to help the free South Vietnamese was merely a cover for their foreign policy of intervening in the USSR’s expansions.
I have found this video that better explains Realist theory by a professor from the University of Ohio.