Well, Russia (then the USSR) started buddy-buddy relations almost since the inception of NK as a country; the USSR was the principal backer and main trade partner of NK from the beginning until the fall of the USSR. For the USSR, cozying up to NK accomplished having their mutual border stretch secured and was a thumbing of the nose to Red China during the Cold War; the only reason China moved into NK during the Korean Was was China's fear NK would fall and China would have a unified, US-backed Korea right on its border; China went in, made their point, and moved out once the ceasefire was in place, leaving the USSR to resume theri sponsorship of NK...
A lot of people seem to believe China has a great deal of influence with NK and nothing could be further than the truth. There is no love lost between China nad NK and that has been a fact throughout the long history of Asia. Just because the various nations are all Asian does not mean there is any ethnic closeness between them. The people of NK hate China almost as much as they hate the US and the Japanese. The Chinese have a great deal of disdain, if not outright hatred of NK. Its like the situation in SE Asia: the people of Laos, Cambodia, Vietnam, Thailand, etc. have long-standing disdain for each other and tend to view their particular cultures as somewhat superior to the each other. It is somewhat analogous to the sort of patronizing attitude of US citizens towards the country of Canada, but with the added baggage of centuries of historical conflict the US/Canada don't have; another analogy would be South and Central America: a common language (exception: Brazil), common foods, common general religion, even a somewhat equivalent cultural genesis, but the nations in SA/CA have a general animus toward each other...
China might move into NK if it appears the end result of a US-NK conflict were to be a pro-US government on their southern border, something they would be loathe to have happen. But, other than that, China has a very strong border security setup to keep North Koreans out of China and would not welcome any refugees if there was a war; China only cares about what happens to NK only inasmuch as it affects their own interests; they are not inclined to spend Chinese blood or resources to blithely defend what they have historically consider an inferior nation...
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