Beside the lazy "a lot of sides"

I know, I felt like Trump -- "on many sides".

But there are a lot of sides to this -- the US, China, Russia (or the USSR), Japan, South Korea, etc -- and it's not really a failure of US foreign policy in particular but a tragedy that has resulted from the self-interested and amoral foreign policy of many conflicting sides. And it's created a situation that is undesirable for all sides but that no one is willing to take action on. For China and Russia, capitulating to the US and renouncing the NK government that they've been propping up since its conception is an undesirable prospect for multiple reasons -- the predominant one being that they don't want a US puppet state on their borders. The U.S. similarly wants a friendly unified Korea, but they aren't willing to take proactive steps towards a regime change because of the potential human and political cost of a war. I doubt than any side wants this arrangement: China has every reason to not want a rogue state with nuclear weapons on their borders, but a unified Korea could be less appealing, and the US wants a unified and friendly Korea, but they are similarly unwilling to act.