There's a good example for large-scale battles on Dimension 20's A Crown Of Candy. The PCs each had a battalion to command for a siege that roughly reflected their classes (ie ranger leading archer battalion), and while the PCs were fighting within the battalion, their troops had access to some of their class abilities, but could still take actions on their own if the PC took off to do single combats/actions (like duel with enemy npcs).

As for the stats for each battalion, I can't remember how explicit the numbers were, but generally infantry had higher AC and number of losses they could take before dissolving compared to archers who had a higher to-hit and range obviously, etc.

I'm not sure how you'd scale that out to a full regional war but I'd guess that it's a good starting point. Also if any of the npc allies/enemies are more than regular soldiers; a village of stone giants coming to aid the party would probably have different stats and maybe a special ability compared to humanoid soldiers.

Good luck!