http://forum.paradoxplaza.com/forum/sho ... ostcount=1
Extrait :
Another major issues from previous versions was the traditional dog-pile event of when you can’t beat them then zerg (Starcraft term for overwhelming your opponent with massive numbers) them. This would cause people, and the AI, to create massive stacks of divisions in a single province to attack from. This tactics was not only historically inaccurate but should have been impossible due to logistics of the area. Paradox, with their experience, tackled this issue using two new features.
First a full logistical system, was introduced, that traces from the units back to its supply source. This system is affected by the infrastructure present in each hex and limits the amount of units you could have in a hex with a viable supply limit.
Second they introduced a unit max front size and an algorithm that measures the border distance between two provinces and determines the size of the front that can be attacked on. This is then applied to both the attacking and defending units. So even if you attacked with 20 divisions, but the front really only has room for 5 divisions, the other 15 will be placed in reserve. The only exception to this rule are Paratroopers as they will add an extra unit to the attack regardless of you having the full front covered or not since it is assumed they are dropped behind lines. You can see a screenshot of a battle on below with the units in combat at the top and units in reserve placed on the bottom.
One more thing to note is officers. Every combat unit represented on the map needs officers. The amounts of officers you produce every day are controlled through various tools such as the Leadership sliders, technological research and educational goals of the government. If you have more units than the required officers this has an effect on command and control which affects your abilities in combat.