Why did US lose to Vietnam war?

Answer 1

Just remember the big 3. The United States:

1. Could not invade the north.

2. Could not use nukes.

3. Could not stop the flow of men/material into the south.

The first two are common sense. The third needs some explaining; nearly all wars are influenced by METT (Mission, Enemy, Time, and Terrain). For this answer, the Korean War will be used: Korea took place on a peninsula, this covers the "terrain" portion of METT. The Korean peninsula was easily isolated by the USN because the US Navy controlled the sea; the enemy had no navy to contest it. Therefore, the US/allies (they called it UN back then) was able to stop the flow of men/material into South Korea even North Korea for that matter. The only way for the enemy to maintain their flow of m/m was across the Red Chinese border; and that took a heavy bombing, it was easy to slaughter the enemy trying to cross that border to resupply. So, Korea was finished up in 3 years, and the enemy failed to conquer South Korea (ROK-Republic of South Korea).

Vietnam was not a peninsula. Vietnam could not be isolated. Without expanding the war into neighboring countries to destroy their resupply lines, Vietnam had the potential to go on forever (literally), men would die of old age, and their sons would have to take up the fight, then their sons, etc. Unless someone decided to quit; the US selected that option. And it was done.