[Mass Combat] Cutting supply lines

[Polydamas]

Also, some of the Mongols' biggest defeats (Ain Jalut, the invasion of Japan) were due to logistics. Their strategy of bringing a herds of horses with them and living off their milk, blood, and meat only worked where there was lots of fodder to feed those horses. They needed fresh arrows, and if they wanted serious siege engines or bridges then they needed to bring up materials to build them.

GURPS Mass Combat pp. 13, 14 has a simple, clear, and reasonable set of logistics rules that a GM interested in the details can expand. If your army is transported to another world, its Land Logistics break with the penalties described on pp. 13-14. If it was supplied through a port and the fleet is sunk, its Land Logistics break. If after the battle for the Dwarf Kingdom it invades the Cloud Castles of the Giants, its Land Logistics break.