I believe China’s strategy is very simple: C1 counterattack into Yunnan freeing up the Burma Road. Consider this on C2 as well if Japan gets nonchalant about that territory. However, as soon as the odds are against you to hold Yunnan, retreat immediately.
Where you retreat to depends on the Japanese enemies and troops. If you think it’s wise to let UKP come at Japan, retreat to Sikang. If not, retreat to Tsinghai or Kangsu.
If Japan leaves your giant infantry stack and fighter alone, roll through their territories with the goal of distracting them.
If not, say your prayers.
As the US, you’ve got to really react to what is going on around you. The one biggest thing to realize is that you must do everything in your power to prevent one side from getting to the victory cities needed for a win. It’s really that simple, but how to stop the Axis varies from game to game. If Germany has taken UK, liberating it should be your first goal in Europe. If Germany is merely convoying/bombing UK to death, send some fighters and destroyers over to bail out Britain while you come in through Africa to ensure that Italy can’t hold Cairo until Germany gets to Moscow. Once you have Africa/the Med, you can choose whether to land in Italy or in France (or even W. Germany or Denmark or Norway or northern Russia). If you’re here, congratulations–you have won Europe.
In the Pacific, you have to be able to count. It’s pretty much that simple. You need to be able to count that Japan can get 5 VC’s without crossing onto the 4th board. Their 6th will be either Sydney, Honolulu, or if they’re a gambler San Francisco.
Simply hold the 4th board (or thereabouts; this can be modified). But, if Japan gets to the point where it has more income than you and ANZAC (India/China will be dead), they win unless a miracle happens. So before this tragedy occurs, count their money. Count yours. Simply ensure that they aren’t too big economically, and if they start growing too big, focus your attention there.
Try to balance both boards while slowly beating the European Axis.