@kravietz Can confirm that the border between China and the USSR in 1989 looked like a war could break out at any minute. I crossed from Zabaikalsk to Manzhouli on the Transsiberian and I'd call the atmosphere at the border at that time... tense. Lots of soldiers on both sides, pillboxes and razorwire fencing on every hill on both sides.
As of what China is doing with Russia right now, my guess is that China wants a chunk of Siberia. Global Warming will force China to go north.