If you leave Russia completely alone, it will turn into a massive, 50 income monster by turn 4 and destroy you.
A lot of people seem to be discounting the possibility of doing a invasion round after round, attacking with all your air but withdrawing from each rounds battle after your cannon fodder is lost.
If the UK builds up properly (men and a few fighters) however, the odds don’t look great on this one and feeding round after round of ground troops into a suicide mission drains too much of your starting forces that should be fighting on the eastern front. With a 100% sacrifice of Ger resources, the battle is almost certain to be won at some point, but the war will be lost because of the investment of so much of Germanys money and time
Destroying Moscow then using that money to turn around and push off the Western Allies is much more optimal than doing it the other way around, because taking London is less rewarding, more costly, and Russia has so many NO takeover targets that it has a chance to stand as equals against Germany.
The USA is usually too weak to cross or stuck out of the war for too long to intervene directly in the UK Ger battle but they are optimally positioned to liberate UK if it did fall, after reviving it the Germans may be weakened enough to succumb to the 3 power crush.