As a war gamer, I find this thread very interesting. But I do want seem like an idiot. I just have some ideas to throw out there. If I repeat anything mentioned before, I'm sorry. Anyhow, this is my first post...so here i go.
One aspect to start off is that sometimes the smalest actions tip the balance for the whole of the battle or campaign. So, think ablout small skirmishes in addition to large, mega, earth shattering battles. In war, soldiers take away from the production of a nation. How would this be factored in.
What are the organization of nations forces including histories of elite units. structure (remember this is fantasy, so not corps levels or such but may go from Roman or Byzantine formations), lyfestyle in and out of camp, recruitment, advancement in rank (if possible), religion in military, Navy, flying forces, magical and clerical forces.
Do the armies move like mobs or in formations. How do they campaign. what is their national, provincial, religious, racial motivation to serve.
Are there different levels of service? How about militia. Hatred towards other countries (I know this has been mentioned in other source books).
My final thoughts, this would be an untimate book. A way to destroy civilizations and raise up others.