Sure, there were external forces at play. There always are. That doesn't negate the fact that they made their own decisions on where to take their society and those decisions were poor ones. As a counter example, I'd say that those who fled or chose to fight the Communist forces made good decisions, at least at a societal level.
Let's put it this way. If the Koreans who fought for Communism in the Korean War could see the situation today. If they could see the thriving success of South Korea vs the tyrannical poverty of North Korea.
Do you think they would change their decisions made during the war? Do you think they'd fight against Communism? I think a lot of them would. They had a chance to do something. They had a chance to battle their leaders and those external forces before the door shut on their ability to do much of anything. Now it's mostly too late. A system has been set up that takes all choice from them.