I have better idea:
Instead starting campaign from beginning of conflict, try to start from "decisive battle" where player side took heavy loses.
For example "chain" of scenarious could be like this:
- escaping from masacre (good for sub because stealth)
- target of opportunity during way home
- securing evacuation (your nation troops from land/islands etc)
- unexpected encounter battle
- returning to base (suprrisingly: without enemy - just for taking breath)
- breaking blockade (during resupply your base was encirclement)
- final battle (against stronger enemy)
No happy end! Because war always costs much more than strategists predict before war
This chain of events allow you to increase difficulity of next missions from low to very high.
Making campaign is not simple. You should have experience with complex single player or multiplayer missions before making campaign. Have you any for sharing?