I should point out that the American landing at Inchon near North Korea is what really sealed the fate of the North Korean army. It threatened, and indeed, cutoff NK from their supply line resulting in majority of NK soldiers either being captured or killed. The loss of NK veterans and manpower made them vulnerable for a UN counterinvasion. Which happened, but then China intervened and did the "reverse SK" to prevent NK being extinguished.

The Inchon landing wasn't even a sure proof plan and was very risky. The terrain was rocky and the weather was terrible at the time of the year when the operation happened. Had the landing failed, the unified Korea would be firmly under the Kim regime to this day. History as we know will have taken a different course.