LARGE SCALE BATTLE SIMULATION IN D&D

Dungeons and Dragons and wargaming have a long history together.  D&D grew out of wargaming.  Fantasy armies are very much a part of D&D and fantasy literature.  There have been multiple products to address the issue of how does a DM simulate a battle between opposing armies without using the individual combat rules which would not work right and would take far too long to resolve.

Basic D&D:  A moderately complex calculation is used to determine a number from roughly 60 to 140.  60 (or lower) tends to be really weak poor quality troops, 140 tends to be elite troops.  (Elves and dwarves are rated highly under these rules).  The two armies face off, modifiers are added or subtracted to each force's number.  (For example defending homeland gives a +10) Then each side rolls a d100 and a chart is consulted.  There are about 15 different outcomes possible in terms of battlefield results.  It has rules that tie-in to player character actions before the battle to give a role-playing option prior to the fight.  It looks simple and probably would work very well with a few adjustments to bonuses and penalties.  The rules work well if you need to quickly simulate a battle and prefer to have random results rather than a DM judgment call about who is the winner.  It also works well with the gazetteers as many but not all of the gazetteers have already determined the 60 to 140 quality number for the armies in that particular realm.

1st and 2nd Edition:  Both have versions of the Battlesystem rules.  I would describe Battlesystem as Risk meets D&D.  There is a lot more dice rolling involved than with the Basic D&D War Machine rules.  Tactical movement on the battlefield is important.  Battlesystem was also written with miniatures in mind. To properly run a Battlesystem fight, lots of miniatures are needed.
 
The 1e rules center around the Forgotten Realms "H series" of modules.  4 modules around a classic good vs evil fantasy battle.

With 2nd edition, the Dark Sun setting was written with the idea of having a significant Battlesystem tie-ins.  Many of the accessories and modules were written with Battlesystem in mind.  Other products (for example the Castles boxed set featuring campaigns against fortresses in Taladas, the Forgotten Realms, and Greyhawk) also featured the Battlesystem rules but the Dark Sun setting was written with having a significant tie-in in mind.  I am not sure that the tie-in was very successful since after a few years they moved away from the Battlesystem focus in Dark Sun products.

 
Birthright:  The Birthright setting also has rules for playing battles that are self-contained within the Birthright setting.  The self-contained rules feature a battle map and 3x5 index cards with unit information on them.  The cards and map are used to simulate battles.  These rules are less complex than Battlesystem but more complex than War Machine.

CONCLUSION: Each of these products has its strengths and weaknesses but overall I think that individually each product accomplishes its goal of simulating a battle between two armies.