Historically UN forces tried to take back Seoul in 1950 after the Incheon landings, allowing the KPA to retreat north and avoid encirclement, and eventually linked up with the "volunteer" force Mao sent into the war.
Can the UN cut off the retreat and destroy the North Korean Army? What effect does this have on PRC actions? They shouldered most of the war effort OTL, but can they take on the entire war effort this time around, where does the line separating the two Koreas end up, and are there two Koreas? Just how decisive is it?