The ground war is one of the least understood and most complex parts of this game. I have not completely figured it out. I do know a few things.
There is a counter for each resistance line that decrements by one each day toward the breaking of that line.
Every turn each unit in contact has a die roll (d100) to see if it is attacked. This roll is checked against a table in memory that is offset by game date. It appears that, for Italy, there is a period where all units in contact (not sure how this is determined yet) have a 50% chance of being attacked. Then the chance goes to 2% (I.E. a sitzkrieg). Then the chance goes back to 50%. I need to figure out the dates when things change.
When an attack takes place two things happen. First the unit attacked is damaged. If it looses it is displaced backwards. Second the counter for that front is decremented if the defender lost and incremented if the defender won.
Win or loose is determined by two die rolls. First a die is rolled from 0 to the current combat value of the unit. Then a die is rolled (I found the size but don't remember at the moment and don't have my notes) and if the result of that roll is higher than the defenders roll then the defender looses. Else he wins. Ergo, combat value is all important.
The current state of a unit has nothing to do with it being attacked or not. Only if it wins or looses if it is attacked. This makes combat value (the sum of equipment) much more important than disruption as it is regained much slower.
Testing has shown that supply line damage does nothing to prevent replacements. Each unit under current TO&E receives 3 Tanks, 3 Art, 9 Inf, and 3 AA. It was believed that the AA was only of a type they already had but recent test show they at least get AA machine guns even if you take all of them away. More testing is needed here.
Supply line damage "may" effect the rate at which disruption is reduced. I don't know.
I believe, but am not sure, that with no win/loss changes the line would break on 5 June 44 (1 day before DDay).
It takes 2 capacity to make a tank, 1 capacity to make an art, and 1 cap to make AA. As far as I can tell Inf appear out of thin air. That shouldn't be right but I don't know.
Some ARM production lines produce for local use and the others produce for the "East".
If you want to actively pursue the ground war you need to:
Make a list of the arms facs that produce for local use. Then make knocking them out a priority.
Attack ground units with an eye to killing more tanks, and art, than are being produced. In the alternative, (what I have been doing) attack a unit till it's guns and tanks are all gone and then attack enough to keep it that way while you work on getting the next one down.
During the times when the invisible ground units are attacking make a maximum effort to keep all units in contact (have to guess) totally weak and disrupted so every attack is successful. That moves the eventual break forward for every attack. YES, The attacks on 29PZGrn in the toe of Italy change the day the Gustav line breaks and if you get a holds up the advance notice then that cost you two days, the day you would have gained and the day you lost.
The Vociferously Verbose