So before the U.S. intervention into the Korean War at one point from the battle maps I've seen it shows that N. Korea controlled almost all of the Korean Peninsula.

And before the U.S. intervened in Vietnam (and when they did so with just advisors) the North Vietnamese seemed to be a far superior force to the Southern Vietnamese Army.

So my question is in both cases why were the North so much more effective at fighting? Was it because of their proximity to communist allies? I'm sure that played a part in it from a supply perspective. But their fighting spirit seemed that much more great than the Souths. Almost like they wanted it more.