We created a Knowledge (Tactics) skill in a D&D game once. Basically you could roll against a DC determined by the DM (factoring in things like the complexity of the plan, and the experience and Intelligence of the opposition), and if successful you could get advice on how to best execute a particular plan, such as infiltrating a building, planning an ambush, etc. It wasn't useful in the "heat of battle", only to set up a planned maneuver, and only covered small groups (ie, an adventuring party). Thus it wouldn't be that helpful in an army scenario.