They were there for like 20 years, suffered heavy casualties, couldn't manage to defeat their opposition and once they left the entire country very quickly fell into communism. And that's exactly what they were supposed to prevent.
I thought it was pretty common knowledge that Vietnam was considered a loss for the U.S.
They left because they couldn't manage to defeat their opposition ?
If American forces had it their way, modern Northern Vietnam would look like Western Sahara.