Don't know if this is in time to help you or not, I hope so.  The Soviet Union and China, while both communist nations, had argued about their borders for decades by the 1980s.  Soviet troops had occupied Mongolia, Manchuria and North Korea in the closing weeks of World War II in Asia.  Since then, both countries have disputed those territories, but particularly the border between Mongolia and China, and present day Kyrgystan and China.  There are resources and farmlands in those areas both sides wanted.

In the modern day, those two countries are independent from Russia, and the border disputes not as intense.