Japan has the ultimate goal of diminishing and or destroying both the Chinese, and UK armies in the pacific. Japan will never successfully take mainland US (unless of course you are playing an inexperienced player), and occasionally they will make a push for Sydney, although they don’t pose a very serious threat. Japan will make most of its income from the “Money Islands” and mainland Asia. The sacking of Calcutta places Japan in a great strategic field. By eliminating the UK in the pacific, Japan can then focus on its navy, and protecting its assets, while expanding its expeditions to either Hawaii, or Sydney.
Now China plays an extremely important role during this entire process, as China can delay or even prevent a Japanese mainland conquest. China as everyone knows plays the game through gorilla warfare tactics. Eventually, this cannot be sustained, as they lose the Burma road, and cannot collect enough IPCs to effectively defend the Japanese advance. Japan will either decide to crush the Chinese forces, or bypass them (while holding on to the Yunnan territory). China however has another option as outlined in this thread. Russia can effectively defend China in the Yunnan territory, resulting in a flow of Chinese units continuously turn by turn. Even assuming that China has only three IPCs, with a 6 bonus, they can place 3 units every turn. Russia can afford to use a few of its units, assuming there is a large number of Allied aircraft landing there to support it. It has also been pointed out that the Russian aircraft can make it back to Russia by turn 6 giving Yunnan 2 turn of additional defense. This gives China enough time to build a significant amount of infantry.
Now I say this because Yunnan is a vital territory for both Japanese and Allied forces in mainland Asia. If Japan retains control of it, it has a forwarding base for its units to make a push in any direction for the conquest of mainland Asia. It also chokes China from IPCs which eventually dwindles the already small army. If the allies retain control, China can begin to grow, and effectively keep Japan from taking many mainland IPCs. It also provides a territory for the UK to land planes, and to make an effective push for coastal territories.
Obviously using Russian forces in this way will not work every time (as you cannot predict the rolls of large battles). I will probably use this strategy once in a blue moon to change things up. The last thing I want to touch on, is that once these Russian units move into Japan on turn 1, they will be declaring war on Japan. This puts the Russian 20 in an interesting situation. They are most effectively utilized by defending. Make Japan attack them. Japan knows that the Mongolian pact is a bust, ensuring that they wont spawn 6 additional Russian units in the north. Even if you lose all 20 units, you are delaying an inland push by the Japanese for at least 1 turn, and you should* kill about 6-7 units assuming your rolls fall within the odds.
Of course this is just my opinion, and like I said, I will use this tactic once in a while. It should be effective enough to stop an ill-prepared Japanese.