No, it exists because 1) China shoved back UN forces from the Yalu and 2) their allies, now in charge of everything north of the 38th parallel, aimed a bunch of artillery at Seoul in case there was ever a thought of the ROK and its allies reunifying the country through force, or any other mechanism the Kim family didn't approve of.

The nuclear part is really just gilding the lily. Of course, China and Russia have helped North Korea circumvent the sanctions that were supposed to punish them for their nuclear program, but it ultimately all goes back to the Chinese support of Kim Il-Sung during the Korean War.