Played online yesterday and generally kept the Soviets out of China. No Japanese DOW. I didn’t like the strategy much because it meant I didn’t have enough troops to hold Yunnan or at least inflict huge losses on their air force against a J3 attack.
With a G2 DOW, whatever troops go into China aren’t coming out to defend Moscow against their initial spearhead attack. In theory, with a G3 DOW, planes can fly back to reach Moscow USSR5. But you have to assume that they aren’t.
Two soviet mechs/tanks provide an escort for the Chinese fighter to sit in Sikang and not need Chinese troops to be stuck on escort duty end of China1. Problem with that being that you can’t position to take Persia USSR2, assuming a G2 DOW. If the UK are taking Persia that isn’t such a problem though. You can also move the two inf in Suiyan down to Shensi and use them as escorts but I like to hold Suiyan (with soviet planes). Forces the Japanese down to Hopei, which they would have taken anyway.
In general, I find with a few soviet troops and a UK2 DOW, you have enough force to make it too costly for Japan to take Yunnan. Obviously, I am assuming no J1 DOW. That would make it a cinch to hold Yunnan for a few turns at least.
For the J4 attack, you can fly up to 7 more planes in (2NZ, 4UK, 1Chinese) so not likely to lose it, even if Soviet planes fly back to the USSR.
Without Soviet or bid help, the Japanese would only have lost 2 planes or so attacking Yunnan. They can accept that, even if they don’t take the territory.
So, I’ve discovered by experimentation why I’ve adopted Soviet help in China as a standard strategy.