ISIS isn't a state and it's not your typical insurgency.
It's much more interesting than that.
ISIS is a marketplace -- a freewheeling bazaar of violence -- and it is rapidly expanding.
So far, it's been very successful:
- it operates freely in an area bigger than most countries (and it has lots of oil),
- it has been attracting the participation of a growing number of organizations and individuals, and
- it's financially successful and self-funding (it's already made billions of $$ from oil, crime, bank robberies, and more).
This success is due to the fact that ISIS isn't trying to build a "state." It's not a government.
It's a bazaar in an autonomous zone. It operates outside of the global system. It doesn't want to be a state (which would make it vulnerable).
This bazaar was built for one purpose: perpetual expansion and continuous warfare.
To keep things running, ISIS offers a minimalist, decentralized governance. Day-to-day life is governed by a simple, decentralized rule set: Sharia Law.
Participation is open to everyone willing to live under Sharia and able to expand the bazaar to new areas.
The strategies and tactics ISIS uses are open sourced. Any group or individual can advance them, as long as they can demonstrate they work.
Weapons and other technologies needed for war are developed, shared and sold between participants and the pace of development based on previous examples is very quick.
Making money through criminal activity is highly encouraged. Mercenary work is encouraged.
All of these attributes (and more) make ISIS hard to fight (something similar to this fought US forces to a standstill for four years in a much less advanced state until Petraeus started using a strategy similar to this).
It also demonstrates that global guerrillas are the cockroaches of warfare. Once they become established, they are nearly impossible to get rid of.