The U.S.'s "entire post-WW2 force" was not the juggernaut you make it out to be. By 1950 the army that fought WW2 had been demobilized; what remained was a small number of career officers and a mass of conscripts with sub-standard training who did not expect to face a hot war anytime soon. The conventional wisdom among American policymakers was that the atomic bomb had made ground forces obsolete, so keeping up a first-class Army was not a priority for them. Moreover, any serious risk of war was assumed to be coming from the Soviets, so what good forces the Army could muster were clustered in Germany and Western Europe to defend NATO.
The North Koreans' early successes were due to the element of surprise they seized with their invasion of the South, the unpreparedness of the U.S. forces in the theater for a major conflict, and the logistical difficulties of reinforcing them due to the long distances between Korea and the mainland U.S. Despite these handicaps, U.S. general Walton Walker managed to halt the North Korean advance, and MacArthur turned the tide by landing behind North Korean lines at Inchon. Once that happened the NK forces fell apart.
I'm also not sure what you mean about "NK was defending against US with no meaningful manpower support from USSR/China." When it became clear that the North Koreans had collapsed and could not prevent MacArthur from reaching the Chinese border, the Chinese sent large numbers of their own troops down to do the job for them, and they did it well. If it weren't for Chinese intervention the North Korean government would not have survived to see 1951.