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 country--tenuously democratic (but with rigged elections)--from becoming part of the communist north. At the time, the U. S. had a treaty obligation under SEATO (Southeast Asia Treaty Organization) to help the South Vietnamese, and our assistance was "requested" under SEATO. In addition, when we believed that the North Vietnamese fired on U. S. ships in internation waters, we believed we had justification for increased military involvement.
As our experience in Vietnam unfolded, it became obvious to many in the United States--both military and civilian--that we were in the middle of a civil war between the North and South Vietnam. No matter how many troops we sent to Vietnam, our enemies were willing to fight and die to expel us no matter how long it took .
Another aspect that was unique to this war is that we generally did not take, occupy, and control land--our strategy was to kill the enemy. We did not ever have sufficient troops in country to occupy substantial parts of the country. Most wars in which we have been successful were won by taking and occupying land until the enemy has no room to hide. In Vietnam, we ignored this fundamental rule, and this helped eventually to insure our departure from Vietnam and the success of the North, which won the war by taking and holding territory until it had conquered South Vietnam.