Remember, Korea had been handed over to Japan as a colony by the United States after WWI ~ Teddy Roosevelt negotiated the deal.
Japan then used it as a colony. In fact, all those nastyassed guards that gave Japanese POW camps such a bad name? The guards were Koreans! They were pi$$ed a lot~! I don't know how any Allied troops survived the camps.
For some strange reason we split Korea into a Soviet occupied zone and a US occupied zone. The Russians then armed and financed the indigenous Communist forces in their zone. Intriguingly the US was busy rounding up "nationalists" in Korea and handing them over to the remnants of the puppet government ~ (and we can all get in big arguments here about whether Rhee was pro- or anti-Japanese so I won't get into that at all). The nationalists fled the Southern Zone to the North, and shortly all sorts of problems happened.
We had not provided sufficient military force to our Korean zone to secure it.
That's the start of the war ~ most folks end up more concerned with the "conduct" of the war, and that involves the Democratic party betraying the troops in the field. Of course that happens every time.
I don't think you can blame the UN for the Korean War. Blame Harry Truman and the guys who forgot to secure the LZ.
In fact, all those nastyassed guards that gave Japanese POW camps such a bad name? The guards were Koreans!
My Grandfather on Dad's side said they drew straws to see who got to kill their guard (he was a slave at a Hitachi copper mine on mainland Japan) after the second bomb but an Army Leutenant with legal experience talked them out of it by explaining that they could be charged with murder since hostilities had ceased...