Why was the vietnam war such a different conflict that the united states had ever experienced up to this time in history?

The U S failed to recognize that the Vietnam was a civil war and most of the wars in which we engaged most notably WWI and WWII were wars based on one country's domination over another. Ostensibly we fought the Vietnam War to keep the southern part of the countrytenuously democratic but with rigged electionsfrom becoming part of the communist north. At the time the U S had a treaty obligation under SEATO Southeast Asia Treaty Organization