North korea didn't believe the UN/US (i.e. Non-koreans) should be deciding the fate of their country. (I am in no way defending the current North Korean regime).
I completely understand a sovereign nation attacking a 'puppet state' in order to unify it.
(to respond to the grand parent as well) If the Russians were 'boycotting' the UN, then it wasn't the UN, it was just "The Allies".
But that makes the POWs change of attitude less brainwashy and more 'informed' in my opinion.
Say what you will about the US, but I think the results of a Stalinist / Maoist mashup speaks for itself. It is and was pretty obvious who the agressor was in Korea -- the Soviet tanks rolled into North Korea weeks before the Japanese surrender specifically to grab Korean territory. The Northern attack on the south was very clearly a land grab.
Soviet Russia was a founding member of the UN.
An excellent understanding of the UN is in Churchill's "THE SECOND WORLD WAR" books.