When you put it that way it sounds obvious, with FOW it is often not clear what condition the unit being attacked is in, but I will watch out for this in future.
Actually, that's an interesting question. A unit will not attack unless it thinks it can win, but if YOU can't see the other side's unit stats, what about your unit that's doing the attack? If THEY can't see it either, how do they know they have a chance of winning?
I see a few possibilities:
1) Your unit on the field has full information whether it's passed to you as the CinC or not.
2) Your unit on the field only sees something like the strength and maybe the unit quality and makes its decision from that.
3) Fatigue doesn't make that much difference in the basic calculations, although it may make more difference in advanced calculations.
4) Fatigue isn't measured the way you think it is. In this case, 90% fatigue might mean "90% of the way to being fatigued" and has no effect until it reaches 100%. This is my first assumption when I see something like a high fatigue unit doing an attack. This is not unreasonable, actually, but it's not necessarily intuitive.
5) The effects of fatigue aren't the effects you expect. You don't get to retire from battle just because you're tired, for example.
My best guess is that the units know if they have a chance of winning and will attack, fatigue from both sides factors into this calculation, and once enough units decide they can't win the corps commander will go into a status that causes him to retreat... which is pretty much exactly what you want. If units were pulled out of battle simply because they appeared tired, people would complain that they couldn't get their corps commanders to attack.
I will agree that there are cases depending on how the calculations are made where you'd want a unit with higher fatigue to go into a defensive reserve mode; for example, if fatigue was important to the calculations and your unit didn't know the fatigue of the opposing unit. I'm assuming that if they don't know, they find out quickly enough and stop attacking.
Hook