Key word is the Allied Navy. Now granted that you can pull all available troops in Europe and hit Karelia with a super stack, you still have a good chance of winning even against 18 inf, 3 ARM, 2 ftrs. However, there’s no way that the surviving force can poke through Russia on G2, especially if the Russians have reinforced entirely with inf. This is where the Allied Navy comes in, when you busy building ARM for an invasion force and have to wait for supplementary inf to arrive on time, chances are that UK and USA will be hitting you in Western Europe or Worst Yet(!) Germany itself – game over.