Since this is a game, I like to know the rules.
The AI is not subject to supply, apparently. It it subject to:
Imagine coaching a football team, and discovering that the other team's offensive line will all be driving fork lifts. Well, even the best d-linemen aren't going to stop fork lifts. They can be beaten, but by digging holes in the field and letting the forklifts fall in.
It might work, but it bears no resemblance to football.
I submit that building an AI that simply isn't subject to the limitations above is not only frustrating but bears no resemblance to the actual campaign.
Whether it is a challenge to beat the AI is a different question. It is Aug 21 and I have destroyed 19 German divisions. Maybe if by October I have killed 60 they will slow down. But the way to beat them isn't to be on defense in good terrain and wait for supply, fatigue, and weather to do their work.
Instead, stay on offense posture, goad the AI into unwise advance, isolate, and repeat. This is frustrating because I spent all the time an effort to switch to defensive posture, etc, which is no good against an enemy that never fatigues, always in blitz, and without need of supply or command.
I am also not sure if such a gambit, being so bizarre, would even be called fun.