Frankly, I'm too lazy to find a citation, but IIRC (99% sure) revelations from Al Qaeda documents have included that they knew exactly what they were doing, and the plan was to strike fear and cause economic costs – they viewed it as a response to ever-decreasing availability of funds and a recognition they didn't have the backing of enough civilians or soldiers to do something as ambitious as found a nation.

ISIS' founding logic was Al Qaeda failed, and surely that was because it wasn't ambitious enough, and not pure enough. The solution was to rebase on fundamentalism and use the remnants of the Iraqi army to create a safe space for a caliphate to rule.

Of course, I'm offering absolutely nothing to back my assertions here, so it's fair to ignore this. :)