It's always a tricky one. One idea that has occurred to me. The Bush War lasted from 4 Jul 1964 – 12 Dec 1979. One simple way of doing things would be to invent a province. If you're playing solo you could pick one side and allow the other to be created and driven by dice. If you've got real opponents, even if they're only on line, they could lay down strategy etc.
The allow things to follow the same general timeline of the war, with the insurgents getting more men and supplies in those periods when this happened, and the military probably being slowly bled down.
What you could do 'as a way of keeping score' is for both insurgents and military, have a system where, when somebody looks competent, they get whisked away to the main theatre and they're lost to you and you have to grow new leaders (even new units)
But each time this happens, for the Military the war lasts an extra month. The people you trained had an impact.
If it happens for the insurgents, the war ends a month earlier as they make their contribution to the greater victory. You might to make it three months rather than one.
But then you're seeing a result of your victories and both sides can 'win'
Success on the ground could be measured in different ways, victories in skirmishes being one. But you could also define a victory as '6 months without a terrorist incident in this town'. This gives both sides something to fight for and forces the insurgent to attack or hand over free victories. Similarly there might be one for the insurgents, 'have your people living in a camp in this area undisturbed for six months' which means the military have to do their patrolling, not merely sitting there waiting to defend the towns :-)
