RPGaDay 18 – Favourite Game System: The War Machine Mass Combat System

RPGaDay prompt 18 is “Favourite game SYSTEM”. I don’t know if it’s my favourite system; maybe 7th August 2022 prompt captures it better: ‘Describe a cool part of a system that you love’. I’m going to talk about the War Machine Mass Combat System from the Companion set of the BECMI boxed sets, which was then built upon by all the Gazetteers.

In the Companion Set of the Mentzer BECMI D&D boxed sets, it contained rules for Mass Combat, called The War Machine, which are still the best set of mass combat rules to fit into D&D which I’ve come across. They balance acknowledging the various factors which can affect a battle – troop quality, numbers, tactics, terrain, fatigue, and of course random luck – without getting into excruciating blow-by-blow detail of every attack of every combatant.

I can’t give a better introduction than the book itself:

The “War Machine” is a system of game rules design to resolve large battles in the [BECMI] D&D game. The War Machine will work with any number of troops; it is recommended for any group with more than 10 combatants. …


  1. All troops have a “level of quality” that can improve or drop with time and experience
  2. Many other factors such as terrain, weather, etc., besides quality of troops influence the outcome of a large battle.
  3. Luck, good or bad, can influence combat results, whether in a single combat or a clash of armies.
  4. A character knows how to survive in the D&D world; the player does not need to know the tactics of war.
Dungeon Masters Companion p12

The system has four steps, three of which can be calculated in advance:

  1. Calculate the Basic Force Rating of the force – the fundamental rating of quality
  2. From this, work out the “troop class” or overall quality of a force – this is a simple lookup table
  3. Calculate the Battle Rating – this takes into account additional factors which can affect the effectiveness of the force in battle, such as how much of the force is mounted, can fire missiles, can use magic and/or spells, and move particularly fast. These modifiers can lead to a Battle Rating more than double the Basic Force Rating.
  4. Having calculated the Battle Rating for each force, you are now ready for battle!

For simplicity, the system requires an equal number of forces on each side, with a side with fewer units having to divide itself into multiple sub-forces (with the same Battle Rating) each facing off against a different foe.

Battle then takes the Battle Rating of each force in the opposing pairs, adds various modifiers for relative numbers, morale, environment, terrain, immunities and fatigue (where appropriate), and then each player/force adds a d% roll. The higher overall roll wins, and the difference determines the results – how many of each force were killed/critically injured, whether each force becomes fatigued, and where the forces end up. A difference of 91 or more results in the losing force being completely routed – it no longer exists as such, and any survivors start to reappear at home 1-10 weeks later.

The War Machine adds various optional rules which can affect the result or future results as well. Tactics affect either the Battle Rating or the number of casualties for each side. Mercy allows the losing side to recover more casualties, but impacts any battle the two forces have in the next year in the favour of the force giving mercy. And various character actions can affect the result as well, things like obtaining information (or misinformation, for a penalty…), attacking with surprise (maybe because sentries have been eliminated), a leader being “removed”, or a PC attempting a heroic (read “risky”) action in view of at least 10% of their force (which backfires if they fail…) . These character actions lend themselves to being a normal D&D adventure prior to the battle.

Example battle

For example, the Drippin’ Blades come raiding from Red Orcland into Darokin, and are met by an army of the XV Legion out from Fort Nell.

Drippin’ Blades: Located in the Sacred Cavern, Troop Class Average, BR 80; 75 orcs with bows and axes, 25 flying orcs with spears and nets, 30 goblins with hatchets, 15 gnolls with pole arms. Total 145 troops.

XV LEGION 1,160 Fair troops divided into 5 armies of 290. MV 4, BR 73

The XV legion have scouted ahead, and prepare to meet the Drippin’ Blades at Dragano’s Valley, a cutting where the route passes between two steep hillsides. They get there in advance and arrange themselves on either side of the valley, ready for the raiders. The raiders are marching blithely in, horns blowing fit to wake the dead and scare any potential victims witless, when they spot a careless outlier.

Situational modifiers:

  • We have 290 defenders against 145 attackers. That’s 2:1 in favour of the Darokin legion. +30 to the Legion
  • The Legion are in their liege’s dominion: +10 to the Legion
  • This is a regular occurrence. The Legion have beaten the Drippin’ Blades before: +10 to the Legion
  • The raiders are on the march: +30 to the Legion
  • The Legion have the advantage of higher ground: +20 to the Legion
  • The Legion are trying to attack with surprise, but there is only a 50% chance of success and they rolled 84. Their cover was blown. The surprise is lost so they don’t get the +40 for a surprise attack. +0.

That gives an effective BR of 80 for the Drippin’ Blades, 173 for the Legion.

Both sides roll d%.

  • The Legion has an appalling roll; it’s not their day: 03, giving them a final score of 176.
  • The Drippin’ Blades roll a 76, giving them a final score of 156.

Despite the appalling roll, that’s a difference of 20 in favour of the Legion. They are the winners. Looking up the results:

  • The winners (the Legion) hold the field and the losers (the Drippin’ Blades) must retreat
  • The winners (the Legion) have 10% casualties, the losers (the Drippin’ Blades) 20% – both lose 29 killed or wounded
  • The Legion are not fatigued, the Drippin’ Blades become moderately fatigued. This will count against them (-10) if they fight again before they get the chance to recover.

If the Legion had managed their surprise roll (50% chance) and rolled better on their battle roll – say 52, they would have had a base of 193 and a total of 245, giving a difference of 89.

  • The Legion not only hold the field, they manage to advance up to 3 hexes, while the Drippin’ Blades must retreat 2 hexes. If the Legion had rolled a couple of points higher, the Drippin’ Blades would have been routed
  • The Legion again have 10% casualties, 29. The Drippin’ Blades have 50%, 73.
  • The Legion are not fatigued, the Drippin’ Blades become seriously fatigued. This will count badly against them (-30) if they fight again before they get the chance to recover.

Since the Legion have managed to advance as much as the Drippin’ Blades have retired, and the raiders are now seriously fatigued, it’s very much worth them attacking again.


The two forces were roughly evenly matched in terms of quality, but the Legion outnumbered the raiders and used superior tactics to give themselves a decisive advantage. And even though luck went against them, it still gave them enough of an advantage to win.

If there are PCs involved, they don’t get injured or killed in the battle – that can only happen in a normal adventuring session. They may try to influence the course of battle in skirmishes beforehand – reconnoitring to gain information can give up to 50 points advantage if they manage to turn a traitor or steal a detailed plan, taking out the force leader in a sneak attack gives 30 points advantage, or 10 points for a lesser officer, and defeating a huge fearsome member of the opposing force can give up to 20 points advantage (of course, failing gives an equivalent disadvantage…).

I like the simplicity of the resolution mechanics, and the way good tactics can make a difference, while still allowing plenty of opportunity for luck/dice rolls to play its part. The troop ratings would normally be calculated in advance (as in the example), which means the focus is all on the tactics and relative force strengths. And a single battle takes a short enough amount of time that various manoeuvring and multiple battles can be resolved within a single session, or a battle plus other action.

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