Not sure why people are arguing that somehow this was China or the Soviet Union's fault. Both NoKo and SoKo wanted to unite the country. In fact, SoKo had 0 tanks because the US was worried SoKo might do something with it if they did.
By the time UN was deciding to cross the 38th parallel, the NoKo forces were fully routed. China warned publicly that if UN forces crossed the 38th parallel, it would have to take matters into its own hands. MacArthur thought this was just a bluff, and even if it wasn't, there wasn't much the Chinese could do to fix the situation.
Again there was no direct contact between the UN and China. Contact was by proxy.
It’s true MacArthur didn’t think it would happen heck he probably thought he could win if it had. Yet then again remember the time here other than massive numbers of manpower China was a backwater. Western nations viewed it as a puppet of the Soviets. Whom was thought as the big bad. Yes there was a degree of racism in that.
Both Kim and Rhea were Korean Nationalists. Both foresaw what he have today that if the two states remain separate they would culturally divide.
Much like the KPA had been in its push to the south the UN in the push back north was quickly out running the lines as the KPA either stripped off there uniforms and joined the civilians joined Southern troops or retreated north.
The Division it’s self was at the heart routed in the means by which the war had ended. The Soviets at the end of the Second World War set to establish satellites across where their troops took control. By blustering existing communist parties or implanting them. That’s the North.
The US set about trying to form Democratic republics with self rule. Which isn’t exactly what happened but was the aim.
The modern mindset is to stop at the 38th. Yet that wouldn’t have ended the conflict we know that now because of what did happen. The Soviets and Chinese came in. Had the UN stopped They would have come in and basically occupied the DPRK rearming and preparing for round 2.
Why? The Soviets wanted a buffer state to keep the west out.
We see this in Eastern Europe.
The CCP was simultaneously launching its own Imperialist expansion into the Western China. With Xanijang and Tibet.
They wanted a buffer and satellite. We know this as post war Kim was purging pro Chinese factions. Given the drive that had pushed Kim he would have done everything in his power to repeat the drive south. So it would have ended the same way. Or worse a second Divided Germany in the East.
Again that would have been the modern mindset which basically ended the war the same way we know it did only longer out with possibly higher death tolls. For the then mindset I point to the end of the Second World War. Once the allies had the German forces on the run on 44 were they supposed to stop at the German border?
For the UN to stop wasn’t realistic as it only would have prolonged the conflict. Kim might have tried to rally the forces and regroup but couldn’t do that without help from the Soviets or Chinese or both. That would have resulted in a failed anarchy of the North, or Soviet intervention resetting the table to June 1950.IE why have to fight the war all over again? It might also have encouraged the Soviets to try another blockade or action in Europe which was still a hot spot. The Berlin Airlift had ended only 18 months ago.
Stopping at the 38th might have been debated but was never going to happen. No matter how you want to spin it.
Once the Soviets and Chinese green lit the invasion of the ROK. The second the KPA started retreating their was only one outcome the Russian and Chinese intervention. They had to intervene to show that they were a power in the Cold War, to prevent the potential of Korean reunification under the UN on both their boarders. By either surrender of the DPRK, collapse of the DPRK internally or occupation by UN forces.
Neither Stalin nor Mao could allow it. They could only accept complete victory or return to the status quo.
IE neither side could not cross the Rubicon without loosing prestige.