I'm preparing for a live session of a 3.5 game, and in that game there may be a mass battle. I'm therefore going through the various mass battle rules I know for 3.5e and 5e.
If you want a more detailed review of any of the following rule-set, please leave a comment.
Open Mass Combat System
This system is found in the
Quintessential Fighter by Mongoose Publishing from 2001. It is very
similar to the rules presented in the Miniature's Handbook, except they
focus only on d20. Unfortunately the rules are made very simple, lacking
many of the options presented in the Miniature's Handbook. I've tried
to run a mass battle using these rules, and it was quite unbalanced,
making initiative a crucial aspect of it. For the 3.5e game, I've
decided to try and use these simple rules as inspiration, so to run the
Miniature's Handbook rules in the same manner but with more options.
This book is an official supplement for D&D 3.5e from October 2003. It is intended to compliment the D&D Miniatures Game. There is therefore a lot of mixed and missing information in this book, but it does focus mainly on mass combat. This book should only be used as a guideline, which I've done for my 3.5e game. As a short example, I created a large battlefield, the edge of a town lying on the southern banks of a river. Elementals will come down the river from the east and attack the city, and the players will have to defend it using whatever forces they have been able to marshal before the attack. I look forward to seeing how the battle goes using this set of rules.
Heroes of Battle
This book is an official supplement for D&D 3.5e from May 2005. It is intended for use by DMs who want to incorporate large scale battles into their games. I once played mass battles as a player using these rules. Though this book does give a sense of mass combat, it doesn't actually play out the battles. It creates a normal D&D adventure that simply takes place during a mass battle. The players are still not set in command of forces or fight throngs of enemies. This book is a great supplement, but not exactly what I was looking for for my 3.5 game.
When Armies Clash
This set of 5e D&D rules was presented as part of Wizards of the Coast's Unearthed Arcana in March 2015. The rules presented here are exactly what I'm aiming to do with my mix of OMCS and Miniature's Handbook. They present it elegantly, simple, and yet with a plethora of options that puts the players in the center and in command of an army, which is exactly what I'm looking for.