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At some point you will have to explain why the common folk, living in a village without any mass communication or media and without much contact to townsfolk, would care so much as to rebel.
As you are using Nazi Germany as template, don't forget that there was no real resistance to the Nazis during their reign in Germany ...
This is what I see as the most significant issue with the premise of the campaign. In Germany, there wasn't the kind of rebellion as when one thinks of France, Greece, Russia, or many of the other occupied lands. But therein lies the key - occupied lands.
From the way this is presented, it feels like it is the name, or primary, kingdom in which the PCs are operating. To me, that is tantamount to a farmer telling Stalin where to get off - it isn't going to happen. However, if you realign this a little and set the campaign up in a conquered, or occupied, territory, then it may work a little better. Think France (sorta).
Yes, this is overly simplified, but during the real war, the French government fled into exile and the Germans ruled through not only force, but a puppet regime. The British, and later American, governments helped local resistance efforts against the Germans. But they (the resistance) had a reason to fight the Germans - it was their country and their government they were trying to win back.
In your scenario, if this is all the peasants have known, then there is no resistance and no reason to clamor to arms. If this is an occupied nation, then the conquerors are a force to be expelled. Having a government, or heirs, in exile trying to rally support (especially if they had friendly relations before) will lend credence to a resistance effort as it would have not only a reason, but faith, in which to wage their campaign.
Historically, if you look at other resistance groups from that period, like in China and Greece, you can have resistance groups that are actually at odds with each-other, but still fighting to oust the invaders (Japan and Germany respectively). But part of that fighting is also for who will control the region when the war is done.
I add that in there because using that type of premise means the resistance can be a significant threat to the occupiers (say, the group that's not the PCs) but doesn't necesarilly have the mandate of the masses, or the crown in exile. This in turn means the PCs have a wide berth to not only help overthrow the occupiers, but also manipulate things so either the rightful heir regains the throne; they (the PCs) can make a bid for the throne; or they can help the other force gain the throne.
I guess all that is just to say, try using an occupied nation, not a home nation as it just doesn't look like it'd work well.