I bring this up because I'm thinking about running a GURPS game about Mass Combat. In GURPS Mass Combat, there's normally one commander who decides the strategy and rolls the dice, and any other PCs can choose the amount of risk they want to take for significant action that might affect the battle but otherwise aren't making big decisions. My expectation is that for the players who are interested in Mass Combat at all, that the only fun position will be the commander and choosing risk and attempting significant actions will be at best a sad consolation prize. The way to keep everyone interested is to split a single battle with a single commander on each size into multiple fronts, one per PC commander. That way, everyone gets to make important decisions.
Mass Combat has some rules for running three-sided battles and subordinate commands (Mass Combat p40-41). Ironically, they advise merging multiple allied sides into a single unified command, so I'm going to disregard that advice and come up with my own house rules.
|It's the historical solution, really.|
These rules apply when multiple PC commanders are attached to the same force that is campaigning as one unit. These rules obviously don't apply if the PCs are each commander a force that is campaigning separately, as obviously those forces are going to encounter different enemy forces at different times with different reconnaissance contests.
A force with multiple PC commanders has to have a single overall commander and intelligence chief during reconnaissance operations. It only splits into multiple fronts when the battle is joined. After that, each front is a separate battle for purposes of calculating troop strength and class superiority, defense bonuses, positional bonuses, casualties, and the like.
Before the first round of combat, the PCs need to split their forces into individual fronts and assign a commander PC to each front. They can do this however they want: equal split of all forces, assign one player all the cavalry and split the rest, give one player all the units of the same race in a multi-racial force, whatever. The only restrictions are that each PC needs to have enough units to be interesting: preferably at least 10 units. Players need to work out the details among themselves, with the overall commander having final say aside from a GM veto in the unlikely event that people are being jerks.
Dividing Forces at the Start
The GM also needs to divide the enemy force as he sees fit, based on the personality of the overall enemy commander. One enemy commander can be assigned to multiple fronts, or subordinate commanders can be created and assigned, as appropriate. In general, each front should have a proportional share of the total enemy force relative to the share of the PCs' forces in the front, but the GM can make decisions as he sees fit: he can assign all the Marine troops to the river crossing in one front and all the cavalry to another front, if that's an appropriate decision for the NPC commander.
The side that lost the reconnaissance contest divides their troops first, and the side that won responds to that division based on the initial intelligence for the battle type (Mass Combat 30). In the case of a tie, the players split their forces first but can reassign entire fronts after the GM has split his forces.
Fighting the BattlesEach front is its own separate battle, and the decisions in one front have no effect in another front. It's possible for a PC commander to win an overwhelming victory and wrack up +8 in PB on one front while an allied commander is slowly losing ground in another front.
One exception to this rule is the length of the battle round, which is determined by the total size of the smaller force involved, not the smallest force in any given front. So if the smaller force is 1500 elements, then each battle round is 2 hours, even if the smallest force in a front is only 5 elements.
Transferring Troops and Collapsed FrontsAfter any round of battle, troops can be transferred between fronts freely. Troops get the PB of the front they move into and calculate casualties as the worse of the front they left and the front they're moving into. Calculate TS and class superiority normally based on the new troops in the front.
A front is collapsed if it has taken 100% casualties, or has won or tied after choosing the Fighting Retreat strategy, or has completed a Full Retreat strategy. An enemy with a collapsed front MUST reinforce with fresh troops, since there's still a PC that needs to fight. The reinforcing troops calculate casualties and PB normally, to a maximum of -50% casualties for the reinforcements and +5 PB for the other side. A PC with a collapsed front can either get reinforcements, using the above rules, or sit out the rest of the battle in which case any enemy troops in his former front can be moved to other fronts. This doesn't apply if all fronts on a side are in collapse at the end of the same round: the battle is over and the pursuit can possibly begin.
Note that it is legitimate to have multiple fronts Retreat while one front remains fighting a rear guard action to hold up pursuit. This is generally pretty rough on the troops fighting the rear guard.
ParleyAny commander on any front may start a Parley, but the Parley only occurs if all commanders on all fronts agree to it. If a Parley attempt fails, only the commander who started the Parley takes the penalized Defense strategy, not all commanders on his side.
SiegesIn a multifront siege, some fronts may be involved in Deliberate Attacks against Deliberate Defenses (called "siegework") while other fronts may choose different strategies (called "activity"). Split each siegework battle round into 6 segmented battle rounds during which activity can take place. Only roll for the siegework battle strategies at the end of the 6 segmented rounds, and only if the forces involved in the front performed siegework for at least 4 of the 6 segments. If a front did activity for three or more segments, treat any segments that were intended to be siegework as though the attacker had chosen the Attack strategy and the defender the Defense strategy and resolve them all at once after the six battle round.
Example: Rachel, Derick, and Will are PC commanders besieging the Ebon Citadel. Rachel and her opponent choose Deliberate Attack and Deliberate Defense, each, for all 6 segments. Derick starts by performing an Indirect Attack into his opponent's Deliberate Defense on the first segment, then tries a Deliberate Attack against his opponent's Mobile Defense on the second segment, and then both switch to Deliberate Attacks and Defenses for the last four segments. Will chooses Deliberate Attack for all six segments, and his opponents chooses Deliberate Defense for the first three segments, but then chooses to Raid for the last three segments. Derick's Indirect Attack versus a Deliberate Defense is resolved immediately, followed by his Deliberate Attack versus his opponent's Mobile Defense, followed by a single Deliberate Attack versus a Deliberate Defense at the same time that Rachel's Deliberate Attack versus a Deliberate Defense is resolved. Will separately resolves three Attacks against three Defenses and then three Deliberate Attacks against three Raids because his troops were not able to spend enough time making Deliberate Attacks.