One small rock accelerated for a long enough time then steered at a large ship (or moon or planet) would pretty much be the end of it.
Can't really imagine much combat going on when it's a mutually assured destruction scenario any way you look at it.
Most mass entertainment scenarios make sure that the attacking force needs to capture (not destroy) what they are attacking to make sure this doesn't come up.
I suppose lots of tiny enclaves (small hollowed out asteroids) on both sides could duke it out with small ships. Still can't imagine a large enough industrial base to keep things going very long, though. Anything big enough to build ships would just be destoryed.