Answer:
Three fundamental reasons why the United States was defeated in the Vietnam War were:
1- the lack of adaptation to the combat field, in which the Vietnamese moved easily because they were familiar with and used to it, in addition to having various passageways and secret tunnels that allowed them rapid maneuvers in the jungle battlefields that abounded in Vietnam.
2- the lack of experience in guerilla warfare, since the American generals had extensive knowledge of conventional war battles but not regarding stealth attacks and ambushes on inhospitable terrain.
3- the American forces consisted essentially of inexperienced recruits, instead of professional career soldiers, with which they were people who had neither the knowledge nor the will nor the conviction to fight in said war.