Prussia rolls a '2' for attrition, which equates to zero attrition losses this turn. They don't make an alliance roll. Add 1I to Berlin, Russians to Grodno and eliminate the Saxon Cav since France took Dresden. The rules aren't clear about the leader that started with the Saxons. He isn't necessarily Saxon, right? I assume he is left on the map. If not, the moves on the previous turn would have been different since Prussia expected to lose Dresden this turn.
Anyway, the French didn't think about their supply lines enough and now are in jeopardy. The Prussians could, conceivably, cut off one or both of the French's northern or southern forward combat formations, but decide that the cost to cut off just one group is already almost too expensive. But which group? In order to cut off the southern group, containing Napoleon, he would have to succeed on two FM1 moves, which, during the winter, will each succeed only 66% of the time. If he attempts to cut of the northern group, it takes more SP's, but involves an attack by 2SP's of Cav against Hannover (which can be reached from Berlin without an FM), and some other moves to close the pocket. A 1-1 against Hannover causes 1 casualty to the French on any roll except '5', which occurs 1 time in 9. Much better odds than attempting two FM1's. I could attack Hanover with just one Cav SP, but at 2-1, the chances of causing zero casualties goes from 11% to 17%. The Prussians don't have to have a unit left to cut off supply in this case, but, since the Prussians are expected to die anyway, whether they are raiding a supply line or defending behind a river, they decide to maximize chances for success. Hopefully, by cutting off the French, it will create extra attrition losses and force the French to waste a turn cleaning up it's rear formations.
The rest of the Prussians set up a line along the Oder River. Since they are counting to knock the northern French units out of supply, the biggest stack they have to worry about is Napoleon's 6 SP's, making a 2 SP defensive stack enough to prevent an overrun. This allows them to extend their line a little further south, making a French attack on Posen during turn 3 unlikely. The only concern with the Prussians at this point is that they may be losing SP's too quickly. With a couple of 2 SP defense points, it will probably only take one attack to completely eliminate the stack if the French bring any cavalry. Since I don't really have a feel for this game yet, I am not sure if the supply disruption in the rear is worth the easier combat results for the French at the front. Ideally, the French will become weak enough, though, to allow a nice little counter-attack with Russians in the future.
The Russians move west. Both the Cav and Inf formations succeed in FM1 moves.
Prussian cavalry attacks Hanover at 1-1 even and rolls a 12, eliminating the defending unit there. French stacks on the northern front are now cut off.
Here's the main battle area at the end of turn 2: