This is a basic explanation of how the Mass Combat system for Pathfinder works. The system is an abstraction of mass combat, and not supposed to be a simulation of large-scale battles, but you will see that once you define certain aspects of armies, the battles flow quick and easy (Note that the system is simple enough to be used on any D&D-clones).

Armies

The first thing you have to define is the Army Challenge Rating (ACR), that defines how strong as an army they are, usually defined by the number of soldiers (units) and the Challenge Rating of individuals. See Army Sizes table for reference:

Now that you know the ACR of your unit, you can define pretty much every other stat:

• Hit Points (hp): Army's ACR * average hp from a single Hit Dice (HD) of a single creature on that army. Example: An army of 100 foot soldiers (fighters) are ACR 1, and their HD for being fighters are d10s (average 5.5), so they have 5 hit points.
• Defensive Value (DV): This is always equal to the army's ACR + 10 plus modifiers, just like Armor Class. It defines how difficult it is to cause hp damage on your army.
• Offensive Modifier (OM): This is your army's attack bonus, it will be used against the DV of your enemy when attacking them. This stat is equal to your ACR plus modifiers.
• Consumption: This defines how much resources, in Build Points (BP) this army consumes every month or week in the field (weekly) or stationed (monthly). It's always equal to your ACR divided by 2 (minimum 1), but you only have to track this if you are interested in using the Kingdom Building rules. If not paid, their morale is reduced by 2 for every payment missing. Assume that each BP costs 500 gp if you are not using the kingdom rules.

Now, there are also a few other stats not based on the army's ACR:

• Name: This is how your army is called, it helps to identify the group in the field and when you have to issue orders.
• Type: This is the type of a single individual in that group, like Humans (fighters) or Elves (rangers). This simply helps to identify the army as well.
• Speed: This defines how many hexes of overland travel the army can move in a single day. This value can be found on the table for Exploration&Movement, typically two 12-mile hexes per day for your average human unencumbered, or a single hex for medium/heavy armored armies.
• Resources: What resources other than the basic to function this army has, like heavier armor, masterwork weaponry, magical equipment, etc. These have a cost listed in BP and will directlly increase your army's effectiveness in combat.
• Morale: This is how confident the army is, this can be increased by winning battles, having a strong commander and using certain tactics. Morale checks are required to prevent the army from routing, escaping/disbanding during battle (see bellow).
• Commander: Defines the NPC or PC that commands this army. The commander grants several bonuses to the army (boons and morale) and can make the difference between a win or a loss in combat. Each army can have only a single commander, but you can assign a captain (usually an npc) that will not grant any bonuses but will avoid penalties in case the army has nobody to command it.
• Tactics: Defines the tactics that this army can use, the commander can grant his army a few tactics as a bonus. These are things like arrow volleys, shield walls, or hit and run tactics.
• Special: This list special abilities that the army has, like an army of trolls have regeneration, an army of orcs can see in the dark, druids can ignore difficult terrain when moving, etc.

Battle

Combat works by taking turns, which have no actual time frame set for them (I personally use 1-hour periods to keep things interesting), while actions happen simultaneously. Meaning that both armies could defeat each other at the same time.

Each battle is separated by several Battle Phase (turns), which are divided into 3 smaller phases:

• Tactics: Each army has to define a single tactic to use against each other. This is set in the first turn of combat and can only be changed with a morale check (DC 15).
• Ranged: Each army makes ranged attacks against each other before engaging in melee. If an army is not engaged with another in melee, they can remain at distance to launch ranged attacks.
• Melee: This means that both armies are engaged and trying to murder each other. On the first melee phase, the commander defines the Strategy of his army (see bellow). After every turn after the first, you can simply repeat the melee phase if nobody wants to change tactics or there is no other army helping with ranged weaponry.

Resolving Attacks

To resolve attacks, you simply roll your Offensive Modifier (OM) against the target Defense Value (DV), and for every point above their DV, they suffer 1 Hit Point damage, until they reach 0 hp, are defeated and the surviving units rout automatically. This means that you have to actually roll higher than the defender's DV in order to cause damage (unlike attacks against AC).

Example: Your army of ACR 3 has +3 OM (offense) and 13 DV (defense), and is attacking an enemy army that has +2 OM (offense) and 15 DV (defense). Both will roll 1d20+OM against each other DV. You will roll 1d20+3 against DV 15, and the enemy will roll 1d20+2 against DV 13.

A natural 20 still causes a minimum of 1 damage even if the result is bellow the defender's DV.

Routing

Armies that take enough damage to have remaining hp equal to their ACR or less must make a DC 15 morale check at the end of the melee phase or they will rout. This means that your typical 100-man fighter army will try to rout when they reach 1 hp.

When an army routs, any engaged enemy army can make one last attack for free, just like an Attack of Opportunity.

Strategy

The Strategy chosen defines how well the army fights during melee, and to change your strategy after the battle has started requires a DC 20 morale check by the commander.

Note that the OM modifier and damage dealt modifier are not cumulative, Cautious means -1 damage, and Defensive -2 damage. While Aggressive grants +1 damage and Reckless grants +2 damage. The table is not clear enough about this and that has been clarified by the author.

Victory!

Well, this was the purpose of fighting and so, it must have a few rewards. The first one is the bragging rights, you won. The second is that the last army in the battlefield wins a new tactic and their morale increases by 1 permanently, to a maximum of +4 morale gained from victories and the commander may win a free boon as well. Finally, once the survivors regroup, the army also recovers a little hp up to their ACR and are ready to engage another army if necessary.

Expanding the rules

The author of this system, originally for Kingmaker and later reviewed for Ultimate Campaign, Jason Nelson, also has expanded the system on a couple of third-party books, Ultimate War and Ultimate Battle. Along with Ultimate Ruleship (which expands the kingdom building rules), these three books will greatly expand your mass-scale pathfinder experience.