The US lost in Vietnam because they had forgotten their own history. During the US war of independence in the late 18th century the American independence movement was up against the British Army, which at that time was one of the largest and best organized and equipped European forces. It should have been a walkover for the British, but as we know the US ultimately prevailed. In retrospect the US prevailed because to them it was an existential fight - they had their lands and their lives to lose, whereas if the British lost they would simply go back home. Forward to the Vietnam war, the Vietnamese were fighting for their own lands and lives, but the Americans, and particularly the troops at the sharp end, had no particular skin in the game and mostly didn't want to be there.
As Mark Twain said, it's not the size of the dog in the fight that matters, it's the size of the fight in the dog.
The US lost in Vietnam because they had forgotten their own history. During the US war of independence in the late 18th century the American independence movement was up against the British Army, which at that time was one of the largest and best organized and equipped European forces. It should have been a walkover for the British, but as we know the US ultimately prevailed. In retrospect the US prevailed because to them it was an existential fight - they had their lands and their lives to lose, whereas if the British lost they would simply go back home. Forward to the Vietnam war, the Vietnamese were fighting for their own lands and lives, but the Americans, and particularly the troops at the sharp end, had no particular skin in the game and mostly didn't want to be there.
As Mark Twain said, it's not the size of the dog in the fight that matters, it's the size of the fight in the dog.