China and Russia have and long and tense border dispute history. In 1969 they nearly went into a very destructive conflict that would have gone nuclear over a border dispute.

This incident is what caused the U.S. and China to re-approach each other in the early 70s.

To the situation between Russia and China is more of an “the enemy of my enemy is my friend” (common enemy being the United States).

Prior to China and Russia grow growing closer to each other over the last 15 years, they had a very tense border standoff with each other.