Russia's policy of non-involvement in the South China Sea (SCS) is now uncertain after Moscow announced that it will participate in exercises with China in those hotly contested waters this September.
Taking place this September in a yet to be disclosed location, any joint exercises have the potential to alienate ASEAN for questionable gain, and undo Russia's fruitful Asian policy.
How ASEAN will interpret these exercises will depend on where they take place. If the exercises occur near Hainan or another part of the sea that is internationally recognised as Chinese, there should be no real risk. The exercises would simply be a bilateral affair occurring in Chinese territory.
If they were to occur in an area that is only claimed however, such as the Paracel Islands, ASEAN could interpret the exercises as an expression of Chinese claims, with Russian involvement getting them into hot water.

Until now Russia has avoided making firm statements on territorial disputes in the South China Sea, maintaining neutrality while other states have been drawn in. In the wake of the 12 July Hague Ruling on overlapping claims in the SCS between China and the Philippines, Russian Director of the Information and Press Department of the Ministry of Foreign Affairs Maria Zakharova, explicitly stated that Russia was not and had no desire to be involved in the dispute.
Russia has pursued good relations with both ASEAN and China. Russia and ASEAN recently signed the Sohi Deceleration in hopes of greatly increasing trade and other sorts of cooperation. Wide ranging in nature, the extent to which it will succeed is debatable but nevertheless represents an intention to build on present ties.
More concretely Russia also maintains a strong arms relationship with Vietnam despite the recent American intrusion. It is currently contracted to supply a number of vessels to the Vietnamese Navy, including the Kilo-class diesel-electric submarine, vessels which incidentally would be ideal for operations in the SCS.
Russo-Chinese relations are even stronger. The two have engaged in several major military and counter-terrorism exercises, in the Mediterranean, East China Seas, the Sea of Japan, and in the Arctic. They also have ongoing arms deals and joint development programs, such as in the development of 4+ and fifth generation fighter jets.

Most significant is their trade relationship, valued at several tens of billions (USD) is Russia's largest with a single country, and for China is a major source of raw materials including energy. The vast expanse of Siberia to the north of China is increasingly significant in their long-term resource security, and has seen rising Chinese investment in recent years.
The pursuit of good relations with both sides is only possible while Russia is on neither. It is possible that Russia has accepted this and chosen China over ASEAN.
Russia's material relationship with China far exceeds that with ASEAN, and China is fundamentally more important than ASEAN in strategic terms. If Russia needed to pay for greater Chinese support, ASEAN would be an affordable price.
However Russia has nothing to gain from siding with China on this issue. While the two nations often cooperate as a loose bloc, such as through the growing Shanghai Cooperation Organisation, this should not be confused with some NATO-style alliance, even putatively.
Russia has also so far managed to enjoy its relationship with China without needing to pay a high geopolitical price. Perhaps most importantly, Russia in recent years has had considerable success without support from any major power. The reacquisition of Crimea and the operations in Syria were entirely Russian successes. Why does Moscow suddenly need support from Beijing?
If Russia is allowing itself to be drawn onto one side of this dispute, implicitly or otherwise, it is sacrificing much for little gain. Russo-Chinese relations have been beneficial for both countries at little cost; the cost of sacrificing Russo-ASEAN relations will not be commensurate with the gain from China.
The exercises this September will reveal the extent of rationality in Russia's foreign policy in Asia, and whether in this Asian Century the Russian bear will sink or swim.