That's what Americans claim to save face. I'm a Vietnamese American and Asian history tends to suck in other nations. America used that narrative to justify invading another country and then dipping out when it was too hard.

I'm still going to overgeneralize, but here's the actual history they don't teach you. (History is much more interesting outside of school).

From the Vietnamese perspective, they initially didn't want/need American help, but about HALF of Vietnam didn't want communism. Similar to Korea, it was kinda like North vs South. America knew that Vietnam has potential to be another booming nation so they saw an opportunity to "help" Vietnam for profit and claim they're fighting communism in the process. America doesn't just "help" for the sake of good, we're a nation of profit and greed.

During this war, South Korea's involvement and then eventual American alliance actually helped South Korea flourish from all this new money they were getting. When Nixon saw that Vietnam was a losing war, he pussied out and ended up moving on to help Korea since that was the next "profitable" nation; even helping in "fighting communism" during the Korean War. Vietnam took a huge L after Nixon pumped and pulled resources so he partially caused the loss, too. Looking back now, America made the "right" call on South Korea as it's now both rich AND has good US relations.

Most of the Vietnamese population today prefers a less communistic government but not much can be done. Government is just stupid powerful BECAUSE of communism in the first place. Like some long domino effect.

TL:Dr America is a sneaky country and does things that only benefit them. They couldn't benefit from Vietnam so they moved onto Korea in "I'm done playing with you" style.