If a war breaks out between Russia Vs China ..
I am sure Russia will certainly have an ally. Inside Asia it will be INDIA. Wounds from previous INDO-CHINA war 1962 healed not fully in INDIA. Russia is its Major Ally from the start and it is time tested.
India and China also share long border. India will provide Russia with Bases and other logistical support. Then China will have to defend its country at two different fronts. Which I am sure will be a VERY BIG problem for Chinese.
Russia will also have an upper hand because Russian Hardware continues to play a major part in Chinese Weapon Inventory. Russia will definitely know about their weapons Pros and cons and also have edge because spares for these weapons will cease to find place.
Russia will emerge as victor.
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In Reality, There will never be a Russia Vs China since,
Russia and China are trying to form a Bloc against US (also planning to bring in INDIA into it TRIANGULAR Partnership)
Even yesterday OCT 17th Leaders of the Both the country resolved their long standing area dispute concerning a common island.