Every now and then in fantasy RPGs (as well as others, of course) you need to figure out what happens when 400 orcs fight 300 elves. Indeed, if you play Old Schhool D&D/AD&D the 'end game' almost must include large scale battles.
The old Chainmail rules covered this sort of thing, but the rest of the game had changed a fair amount when Battlesystem was released as an update to Chainmail.
In a nutshell, Battlesystem is an attempt to make mass combat in D&D/AD&D something manageable and that can be played out on the table with player choices mattering instead of having the action occur 'offscreen'.
Battle system is very much a supplement! It is not designed to replace or compete with stuff like Warhammer - it is, very simply, a tool for DM's and player to resolve certain game situations.
Battlesystem does its best to be simple and straightforward. All groups are divided into 'figures', or groupings of similar creatures (from 10 creatures like orcs to 1 creature like a giant). These figures are then grouped into units and the units as a whole have a set of stats to describe their abilities on the battlefield. Stats include things like movement speed, armor class, THAC0, etc. as well as morale and discipline ratings.
Battle consists of maneuvering units as groups and combat rolls are made for units as a whole, not by figure or by creature. Rather than being just a statistical game, though, players do roll dice to see how well or poorly units do from round to round, adding a touch of luck to the system. At the same time, Battlesystem is still designed so that better tactics outweigh better luck!
The game is divided into sections (basic, etc.) so that you can learn the rules in steps.
While simple, the rules do cover a wide range of topics and are detailed enough that 99% of situations are going to be covered directly or indirectly.
I will continue with play reports!