I usually play the Axis. What is the most difficult to defend is a combined US and British force near Germany.
American fighters with no land forces means all you can do is add defense and pose a threat against navy, since there is no german navy and Japans navy is out of range you are on defense only. Picking off lone infantry with a stack of fighters is expensive and should only be done to spring allied tank forces.
With US navy help the British get going faster and spend more on land forces and less on navy to defend transports.
The 1-2 punch of the allies is something you have to account for on every territory the allies can hit. The brits might get hurt softening up a target but then the US hits it and knocks out what is left. So you as Germany now are on a leash, get your forces to far away from Germany and you are in trouble. You have to always keep enough back to prevent a beachhead from forming.
Since the allies with have much airpower only a few land forces can do much damage. 3-4 tanks with 4-8 air units can take out 6 land units easily and at an attrition expense the allies can afford.
It does take a bit of time to set up your supply chain so work on slowing down the axis until this happens or playing conservatively until the US can come in to save the day.
The thing to do with any country is to ensure you can threaten alot! You might never carry out the attacks but threaten EVERYWHERE you can to force your opponent to defend alot of places. The allied navy can hit 3-4 spots Germany controls so they either trade those spots or defend them heavily enough you cannot attack. If Germany defends 3 places you are leveraging your force by a factor of 3 in tying up Germanys forces so they are not on Russia’[s doorstep.