“Contain” and “Prevent from winning” are pretty rough words. Full disclosure: I +1’d ColonelCarter’s post.
Japan I feel is a matter of dumping 75-100% of America’s income into controlling. You must do this. The problem with that is what happens with Germany as you do it. Japan getting that last VC for the win is pretty rough before fairly late-game. Hawaii can be turtled up, and ANZAC’s capital can be defended very well. While you do that, Japan tends to begin to seep into the Atlantic theater.
The problem with this question is that its simplicity begins to fall apart by the 6th-7th turn of the game. For the first few turns, no doubt, the axis lead the way and the allies must react. By the midgame, however (turn 4-7), both sides must react to what the other is doing. By the 6th-8th turn, nearly every game is quite different. In that instance, which side do I find most difficult to handle? Well, Germany: as I voted.
Whether this is an artifact of the way I, personally, experience the game, or whether this is a catch-all response, is entirely unclear to me.
I hope my vague words help >_>