I think we already have a mechanic for the that, usually when a province of mine is surrounded by an enemy nation, it loses Morale because of the "Neighbors Penalty". I don't quite understand it yet, but as far as I know if one of your provinces is even bordering an enemy nation it loses morale. As for the troops, it's an interesting concept. It is true that cut-off units start running short on ammo and food (don't know about food since soldiers usually take some from the villages) so maybe a -25% damage penalty for units surrounded by enemy provinces?
- Leading officer of the Training Alliance