

I think it wasn’t so much rivalry as a side effect of the Sino-Soviet split, Vietnam didn’t want to cut ties with the USSR so China supported Cambodia against them.
Same reason China supported jihadists in Afghanistan really. What came out of it was the fall of the USSR (Afghan war wasn’t the only or even main cause but a major one for sure), and China ended up with an extremist state on their borders which exported terrorism into Xinjiang, and the rest is history.
The immense misery of both China and post Soviet peoples in the 90s could probably be avoided if Sino-Soviet split didn’t happen, if they worked together against religious extremists and counter-revolutionaries.
And if the Tito-Stalin split also didn’t happen…

Are we blaming AES now for the shit that happened after they collapsed and got replaced by capitalist states?
Yugoslavia and USSR both ultimately failed and collapsed, that much is true. I believe they could’ve survived if they formed a strong alliance, including China.
But the federalism of Yugoslavia and the national policies of the USSR were both incredibly successful in squashing ethnic tensions. People lived peacefully and most would never think fratricidal wars are even possible anymore.
At least with the USSR, I think things would be much improved if initiatives like korenizatsiya, zhenotdel etc were not abandoned.