Using Battlesystem V1 in D&D 5E

BattleSystem v1 and best.
Accept no substitute!

A brief diversion from plastics on a budget, since I was thinking about what you can do with large quantities of plastics when you've bought them!

Warning, contains basic maths.

If like me you're a bit of a wargamer as well as a role-player, and like me you go waaaaaay back, you may remember TSR introducing BattleSystem. their mass combat rules, back in 1985. It was written by Douglas Niles, also responsible for a bunch of assorted Dragonlance (and other) scenarios and a number of novels. V2 came out in 1989, and had a noticeably different system, which, frankly, I hated. So we'll stick to talking about V1.

If you can find a copy, it's well worth it. Streets better than V2, if a trifle old school in its use of a combat results table, which even then was starting to drift out of the wargaming hobby. V2 moved to a buckets of dice approach, which did require you, potentially, to own lots of d8s for longsword armed units, say, which I didn't like. In this enlightened age I'd probably tweak it to add some command and control rules and initiative based movement rather than the I-Go-You-Go that's in the original rules. But the great thing it did is it allowed you to have lots of units on the table and fight them in groups without having to handle each figure in a unit individually, AND your PCs could get involved while still using the D&D rules for their actions if they needed to, and as such it ticked loads of boxes for me.

However, it is written for 1st Edition AD&D, and as such it uses old-style lower-is-better AC and THAC0 in its calculations: essentially you perform a one time calculation to determine a unit's attack rating, then subtract the target AC and add 2d6, then cross reference the number on a table against the size of damage dice the attacking unit does. This determines how many hit dice or levels of the target unit you kill per attacking figure. You remove defending figures proportionately (remember, for a unit of 1HD orcs, every figure represents 10 orcs and thus is 10HD) and if the balance is more than 1/4 of a figure, the unit gets a wound marker - two wounds = 1 kill.

Essentially, the unit's attack rating is its THAC0 (To Hit Armour Class 0, for those too young to remember 1st and 2nd edition) plus a modifier based on its figure ratio - units up to 4HD/figure are 10:1 (+0), up to level 8 5:1 (+5),  level 9+ 2:1 (+10), PCs and individual critters are 1:1 (+15) - it's a one time calculation when you create the unit.

So, as we said.
attack rating = THAC0 + ratio modifier

In 3E and beyond, of course, armour classes got flipped, so big was better, and AC0 became AC20. To hit AC20 you need a 20 with no modifiers, and your target roll in 5E effectively drops by 1 for every point of attack bonus. So your 5E THAC20 (essentially the same number as your 1E THAC0) would be 20 - your 5E attack bonus.

Your roll on the BattleSystem results table, as we said before, is (in 1st Edition speak) THACO + ratio modifier - AC + 2d6. Time for a bit of maths to convert this to 5E:

attack rating 
= THAC20 + ratio modifier
 = 20 - attack bonus + ratio modifier

So your roll on the table becomes
20 - attack bonus + ratio modifier - 1E armour class of target + 2d6

which becomes
20 - attack bonus + ratio modifier - (20 - 5th Ed armour class) + 2d6

Which, rather elegantly, simplifies the 5E version of the calculation to
armour class - attack bonus + ratio modifier + 2d6

I think we can work with this. :D

The rest of BattleSystem doesn't need much conversion: things still have levels/HD, and attacks you can measure in dice. I may give this a try, possibly with more modern turn sequencing and command and control, sometime and see if it works.