Following a discussion in another thread, here is some information on the War Machine, the system from BECMI (actually the Companion Set, later improved in the Master Set), that I have ported to every single edition of the game except 4e.
I have a file about in the Homebrewery, but unfortunately, the original war machine is still under copyright, see below. That being said, for personal use, I can share things like the excel workbook that I use for computations, etc. contact me privately.
"This is an attempt to refresh the only mass combat system that worked for me throughout all the editions of D&D. It first came out with the Companion Set and was then developed in further expansions of the BECMI rules. It is a fairly abstract system, which - amongst its many advantages - does not require any investment in map space and figurines, it can absolutely be played using Theater of the Mind (TotM). It also scales extremely well, from skirmishes to battles involving thousands of troops on each side. Last. but not least, it gives a large part to PCs who can not only lead troops and choose tactics, but greatly influence the results of the battle by proper planning, spying, and doing heroic actions on the battlefield."
"Due to copyright, I will include the bare minimum from the original text to understand the intent, but if you want to read and understand it fully, you can get the rules either from the original expert set or from the D&D Rules Cyclopedia (which I recommend as it is more complete and updated and also contains some expansions in particular for Siege, but not the naval one - which also works for Spelljammer, by the way) in particular from DRiveThruRPG, here:"
A few more words from a post on another forum:
This system is absolutely brilliant, basically from a few elements about the type of troop that you have in units, you compute a Basic Force Rating (a bit like in Unearthed Arcana), and this never changes for a given unit. You find the Troop Class (from Untrained to Elite). You then add a modifiers (mounted troops, flying troops, magic or magical support) to get your Battle Rating. You then take into account the ratio , morale, environment, terrain, immunities, fatigue, etc. and (and this is brilliant) a tactics modifier, you roll d20 for each force and add the total and it gives you who wins, and how many troops you lose.
To give you some example:
Example: An elven prince has a force of 500 elves, all 2nd level and equipped with bows and longswords. The BFR is 96, and troop class is "Good." One hundred of his elves are mounted on pegasi, and all the elves can cast spells. However, only 12 elves have magical items. The bonus is 10% of 96: 9.6, which rounds up to 10. The prince adds 10 to the BFR eight times: for a. (20% are mounted), c. and d. (more than 20% can fire missiles, and the maximum range is greater than 100'), h. and i. (the entire force can cast spells), j. and k. (20% of the force can fly), and 1. (the average movement rate is well over 100' per turn). The total bonus of +80, added to the BFR, gives a total BR of 176.
Where it is brilliant is the small bit about tactics, because you can choose your tactic and so does the opponent, and when they are revealed, you get bonuses:
Attack + = forceful attack to overrun
Attack = close and combat
Envelop = attempt to encircle foe
Trap = lure foe to an ambush
Hold = stand fast at all costs
Withdraw = retreat rather than fight
There are also other optional factors about whether you want to be merciful, whether the PCs have an impact or not, surprise, effect of the loss of a leader (probably due to PC actions).
It also works really well for sieges, as well as for naval battles. I actually remember a huge campaign in D&D "Into the Maelstrom" where my players managed huge fleets and their tactics at the empire level. Absolutely brilliant, and really simple. I am planning to resurrect it for 5e for Descent into Avernus to manage the battles in Avernus, so happy to discuss further if you are interested.
