If you're only going to list 3, then instead of listing Chinese Intervention, I'd say, US colonialism. French Indochina (i.e. Vietnam) was a colony.

France failed to hold the country together, and we took over because we had the money and weapons. First as advisors supplying weapons and training, then escalating to direct fighting. The South Vietnamese regime was massively corrupt and only held together by our (US) money and support.

There were a lot of people supporting the South Vietnamese government, but also a lot who supported the North, and some who just wanted to be independent.

We could never achieve victory because the country itself was divided, and a huge local power (China) right next door had an escalation advantage we could never match.

When we pulled out, the South Vietnamese government collapsed immediately, because they had no large Vietnamese basis of support. Virtually identical to what happened when we pulled out of Afghanistan last year, and what I fully expect to happen if we pull out of Ukraine.