Suggested solutions typically entail either breaking it down piecemeal until it becomes skirmish-scale again, treating it exactly like skirmish combat just with bigger numbers, or just ignoring it entirely and skipping ahead in the narrative.
None of these solutions feel great to me. Like the concept of Initiative, it's about picking the least worst option.
Here is my take:
If the Adventuring Party fights an Army in straight forward combat, then they lose.
Armies always win.
The largest command subdivision that The Party can engage with in skirmish combat is The Troop. About 35ish enemies.
If the Players, alone with only their party, engage with the main force of an enemy Army then the Players lose. They are captured, killed, or routed. The resources that the main body of an Army can bring will heel even high-level characters in mere seconds. Dozens of arrows, multiple castings of Hold Person, swarms of claws and bites, Counterspells, Teleportations. The Players could cut down a hundred soldiers in six seconds and it would merely be a dent in the body count of the Army. Retaliation against small-scale assaults is disproportionate and decisive.
You tell the Players right from the get-go that if they engage that Army alone then they will lose. You tell them that before them lie foes beyond accurate count, and that they have no hope of victory alone. You do not have enough d20s to properly simulate how many attacks will come at them per turn. (No doubt for some of you this is a bluff - you have more dice than stars in the sky - but I digress.)
If they decide to attack regardless (and admittedly there might be valid reasons for this), then one of the following happens:
Roll for each member of the party.
1-3: Routed. HP is +1 above minimum consciousness threshold. All Spell Slots are expended. All combat abilities with Charges are used up.
4-5: Captured. PC is now a bargaining chip for the enemy army.
6: Killed. Butchered. Corpse dealt with however this army deals with corpses.
If the Players integrate themselves with one Army to fight another, then they share the fate of their Army. Noteworthy contributions to the fight are noted. If Victory is achieved, PCs are left with 1d20 HP left and all abilities with charges expended.
~~~~~
Interacting with Armies
How do Players interact with Armies then?
The answer is: not directly. Players do not directly defeat Armies. Armies get defeated either by themselves, Politics, Rival Armies, or Acts of God (such as a Wish). Many of these things are within the capabilities of an Adventuring Party, especially a high level one, and are far more interesting than trying to mow through thousands of enemy combatants in small-scale skirmish combat.
EXAMPLES OF PLAYERS INTERACTING WITH ARMIES
Cause an Army to Defeat Itself...
- Convince one or more factions within the army to abandon the cause.
- Convince the soldiers to mutiny.
- Delay the Army until the end of War Season.
- Mass desertions
Do Politics...
- Negotiate a peace or demobilization.
- Bribe mercenary leaders to abandon the cause.
- Random and/or kidnap someone important.
- Excommunicate Army's leaders or supports from The Church.
Rival Armies...
- Raise an Army to fight the enemy Army.
- Provide intelligence to friendly Army commanders.
- Convince a dragon to hound the Army.
Acts of Gods...
- Flood a Dam, Cause a Typhoon at a crucial moment, Summon a Tornado.
- Poison the Water Holes, Raid the Army's Food Supply, Burn the Fields
- Cause an Avalanche or Rockslide
- Start an enormous Forest Fire.
- Cause the Army to stumble upon an extinction-level Curse.
Skirmish Level Fighting Units
Enemies: 1d4-1 (2)
Enemies: 2d4 (5)
Contains: A 2HD Veteran
The smallest unit of an Army above the individual: a handful of soldiers with a Veteran as senior.
A Veteran is a tough guy who's experienced combat. A Sergeant/Corporal equivalent. Expect them to act with resolve and decisiveness when shit goes down.
Enemies: 3d6 (10)
Contains: A 3HD Enforcer, Two Veterans
Two or three Teams. Big enough to be a serious threat to an Adventuring Party. Led by an Enforcer.
An Enforcer is a real tough dude who knows his shit. A non-commissioned officer equivalent. They're often a spellcaster or paladin and will command others in the Squad to act intelligently.
Enemies: 10d6 (35)
Contains: A 4HD Officer, 1d3+1 Enforcers, 2d4 Veterans, and Something Big.
The largest unit that the Player-Characters can fight in Skirmish combat without automatically losing. Approximately the largest number of soldiers than a single person can directly oversee at any one time.
An Officer oversees the Troop, and will have direct access to the Army at large through the Army's communication networks: Sending spells, animal messengers, runners, crawling hands that use coded sign language... If the Platoon is engaged then the Officer will consider calling for support or reinforcements. Depending on whether the message gets through and how isolated the Platoon is, this puts a timer on the encounter (anywhere from Rounds to Minutes to Hours to Days). After the timer counts down the Army's arrival is announced and if the Players stay they will lose (see above).
In addition, a Troop has access to Something Big: a military support resource that can really put the hurt on some meddling Adventurers. What this is varies quite a bit between Armies.
Examples of Something Big: a mortar crew, a golem, a trained basilisk, a Counterspell battery, a summoned demon, a tank, an attack helicopter, a legendary Hero.
Troops can be clustered together or far apart. Rarely will they be out of shouting distance from the Officer, though. A Troop can comfortably occupy a dungeon across multiple rooms of a single Level. Any particular Squad or Team can summon the others, each arriving separately within 3d4 combat Rounds.