Strategy for Operation Downfall: Invasion of Japan
Operation Downfall: Strategy for the Allies
The main way to beat this scenario is to hack slowly away at the Japanese possessions. By this I mean first take the southern part of the Korean peninsula (if you play as the Allies, the USSR should handle this). Next, go for Taiwan. All during this time, by the way, you should be using all of your planes to bomb Japanese positions everywhere.
Next, make sure the Japanese southern most island, which consists of 2 provinces, is secure. To do this, land 5 or so divisions of the southern province. Deafeat the troops there, then transport in 10 more divisions. Then, take the northern province.
Take care of the northern most island next. Use the same strategy as above. The only difference is leave a garrison in the southern province because the Japanese can reach that province.
Now it's time to attack the main island. I suggest landing at Hiroshima with 10 divisions. Once you establish a beach head, advance towards the province next to Hiroshima. Speed is crucial in this situation, because it will be difficult to defeat Japan if they're dug in.
During this time, launch an amphibious invasion of the north section of the island. This will catch the Japanese off guard, and you should have an easy victory now. Just capture Tokyo, and you've won!