It could work with a scaled-up
Diplomacy map (covering a country or region in detail rather than all of Europe). Basically, the map is divided into territories, each of which can hold one army or navy. Each army or navy is identical in strength, and you must support attacks and defences from neighbouring territories to try to dislodge (force into retreat) enemy units. Roughly a third of the territories hold a Supply Center, which when captured by a player allows you to support one unit. The players all see the map and can talk freely, but orders are secret and simultaneous; you win by controlling over half of all supply centres.
With a few changes - eliminating armies and navies in favor of one unit type representing simply the ability to project from an area, different win conditions for different groups (some want to control a holy/national city, some want to hurt the occcupier, the occupier wants to hold certain objectives), a custom map - that would work great. The beauty is that the asymmetry would come simply from the map design - the occupier starts with a better position, with a very dense SC zone along a coast or something, representing strategic ports - and abstract out all the details without losing realism or balance.