I rarely use the phrase "game breaking", but that's exactly what this problem is.
This past weekend I finally launched a successful invasion of the US west coast while playing as Italy. I landed there, fought a bloody battle to establish a tiny beachhead, and slowly carved out a small defensible area to be able to stage a larger offensive in the coming months, carefully managing very limited supply.
Then what happens? Germany and Japan hear there's a party going on in the Seattle area. They immediately send at least 50-60 divisions each, which come flooding in like locusts.
My supply quickly drops to nothing, we all lose organization, and then we slowly weaken from attrition. My troops become unable to fight any further. We're not even strong enough to hold the front line. Then before I can even cut my losses and pull everyone out, US troops mount a counter attack to repel the invaders.
The entire operation is wiped out within days, without anyone breaking a sweat.
This invasion was the Holy Grail operation of my game as Italy. Months of strategic planning, many brutal hard-won battles, preparing and slowly conquering for four ingame years just to be able to make this invasion practical (the US was by far my biggest nemesis), and it all goes right down the toilet for a clearly obvious problem that frankly never should have escaped playtesting.
So far I enjoy the game immensely, but this automatic military access situation coupled with the AI completely disregarding fatal supply shortages is incredibly frustrating.
I literally watched over 150 divisions crammed together and sitting idle in mountainous territory while they wasted away, before the US counterattack arrived to mop them up.