The North Korean economy completely collapses, even more than what happened, so that not even the Party or the military is able to keep a lid on things. China finally having had enough of the whole mess cuts a deal with South Korea to not interfere if they invade or even try and use their influence to help convince senior people to change sides if South Korea in return promises to shut down the nuclear programme under UN supervision, remove US forces from the peninsula and head off the potential flows of refugees across the Yalu river. Since the two Koreas are still technically at war the South would legally be allowed to take action, the North is put under military occupation/governorship much like Germany and Japan were after WWII with the borders maintained and population not allowed to move out to avoid a humanitarian crisis. Over several decades they rebuild the infrastructure and economy, to help offset the horrendous costs they set up commercial zones where thanks to a combination of South Korean technology and low wage costs in the North they're able to undercut even China in a number of the manufacturing sectors. After twenty-odd years of building up the infrastructure, slowly liberalising society and allowing short family visits from the South to the North they hold a referendum on either reunifying with the South or ending the occupation and becoming a fully independent state. Whilst there is some ill-feeling on actions taken in the intervening period the vote is in favour of uniting as the Republic of Korea.