Getting started with designing Wargame rules for military fantasy campaigns

For a while now I've been occasionally thinking how cool it might be to run a campaign in which the players are the captains of their own mercenary companies which have all been hired to support an army defending a realm against the monstrous horde of an evil sorcerer or warlord.

Roaming the countryside looking for the enemy, following their paths of destruction, sending out scouts to detect enemy warbands and determine suitable battlefields to engage them, defend towns from raid, secure supplies to feed their own troops, and all of that. Battles would involve each player controlling dozens or hundreds of soldiers and sending messengers between each other to coordinate, with strategy meetings and the questioning of prisoners and locals making up the bulk of the roleplaying elements.

Since you don't really need character stats for this as in a regular RPG, and the main effect of the PCs being present on the battlefield would be to boost the units they are attached to, I think it would probably be a good idea to simply design a complete system from scratch.

Though having never even played a single wargame ever, I've been wondering if anyone knows about any resources I should be checking out to get some idea about what other people in the past have found to be working well for them or not. Anything to recommend?