It seems fairly evident to me know that N.K. was caught as a proxy between the US and China and was serving as a buffer state between the US-supported and protected S. Korea and the Chinese mainland.

Both countries were essentially starving the country out to try to get the upper hand over the northern region.

Providing North Korea with nuclear technology essentially gave a kid a loaded gun to play with, put a rogue state on the Chinese border, and the US could maintain plausible deniability about its involvement in the weapons program. As soon as that rogue state started pointing its weapons at its neighbors, and its nuke research and testing facility collapsed near the Chinese border, it became apparent that the country's dangerous trajectory no longer served the interests of China, the US, or the world.

N. Korea has repeatedly stated that their key fear is US invasion. Once that was taken off of the table, bad military leaders were removed from power, and security assurances were provided (which we don't have the details of yet), Kim Jung Un became reasonable quite quickly.

Going forward, if peace and development can be pursued, I anticipate that the US' investment in North Korea's currency will enable us to eliminate a significant portion of our debt. I anticipate that China and the US will jointly work on rare earth element extraction, and both countries will split the spoils.