I like the idea of peasant armies simply stopping the invading armies from looting.
That is a very innovative idea.
You can loot until they spawn. Then you have to hunt them down in their woods and their buildings and...well...they may get some of your soldiers while you do it. Or you can just go to another region.
It would help to keep small raiding parties from being overwhelmed, and it would give people a reason to move around more.
