Good point and good question. I wonder where ISIS fits in with your question. I realize they are not a state but maybe they are the model of how a band of broke thugs organizes to take over wealth and territory.
Or maybe they started with rich backing?
I think it’s pretty obvious ISIS is getting financial backing from somebody. I haven’t a clue who that might be. There is simply no way major military operations can be self-sustaining in the modern world.
The territory conquered by ISIS is not even close to being wealthy enough to support their military effort. Though the immense amounts of equipment abandoned by Iraqi forces certainly help a lot.