There is something unique about pseudo-religious terrorism as in you can't win. I don't even know what their "victory" conditions are.
I would never attribute decisions to theology and deism, when more mundane explanations are also around. I already excuse myself for the longer post here, but there seem to be some misconceptions:
If you analyze their actions and activities, it is first of all about power over people and you only play by their rules, if you promote their interpretation of their religion as a valid one (It isn't, which many theologians have already pointed out) and their worldly strategy somehow justified by divine law.
As always, a terror organization, even a loose one like the Islamic State, requires resources. It needs people, money and infrastructure. Which requires the Islamic state in first place to exist and appear powerful enough to gain more such resources as they consume. Nobody would join them if they would be just one nutjob out of many. And with power, you can share it among your supporters.
Even for the useful idiots committing such badly planned attacks, the alliance with the IS is useful, since they promise a legacy. I think we all know the human desire of controlling how we will be remembered. And be remembered at all in first place. Many of the attackers were stuck at a point in their life. Even those who went from the west into Syria for the IS have rarely been people who had a plan of life before the IS offered one. Many had been in psychological therapy, too often for depression and suicidal tendencies. People, who had little to lose and too often fall behind in our modern competition-driven society (which is cold and soulless, I have no illusions about it there). They are those, who really appreciate the "fast exit" from their misery that the IS and its network of supporters and service providers offers. Just join us, we will help you to do something that the world will remember - and we will make sure that people will never forget you. The classic Mephisto.
If the IS would attempt more complex attack schemes as Al-Quaida did during their prime, they would consume way more resources by that. It isn't about efficiency there, as you can see. It is a full "cost&risk-avoidance strategy", even if this means stagnation.
And since they are facing some competition among the "true way of Islam" fraction, they can't afford the classic victory condition of not-dead-yet, as guerilla groups can. Just being not dead yet means for them, other groups will take over. Including the mainstream of Sunni Islam.
They need to appear like a major fraction. That is why badly need people committing "lone wolf" attacks in their name, so they are not forgotten. Every idiot pledging alliance to the Islamic State before getting killed is one victory more for the Islamic State, because their brand is kept important, even if their brand identity is suffering. Only that way, they can get donations, volunteers and useful idiots for more such rather futile attacks. Futile, because, as you can expect, not even thousands of such attacks would be enough to defeat a country. And even if a country would declare defeat: I doubt the Islamic State would know what to do then, because it is not planned.
The process will be like in Syria: Some idiot will likely declare himself the boss, fight other rivals claiming to be boss as well and eventually something will get worked out that at least does not look too chaotic. Even though everything necessary for keeping a state running does not exist and will never exist in the IS, because of their selected strategy.
So, what to do to fight them, how to defeat something that will not admit defeat just like Monty Pythons black knight? I think there is only one hard and long way: Don't waste your breath hunt the useful idiots with the knifes. They are just expendable ammunition for the real decision makers, without any training or strategic intelligence. Those will need to be attacked: The social media influencers of terror, the low-profile guys operating their servers, the grey eminences shaping wahabi terrorism from outside the web. And you can attack them best by attacking the fragile trust among them. Their paranoia is also their weakness.
And while you do this, fight despair, give young people a better perspective as the terrorists can. You can't always win a battle for a soul, but winning enough battles will make sure that the Islamic State will become more desperate for attention, use riskier strategies and force the people in the background closer to the front line. It likely means, that things will become worse before they will become better, but you will notice it, when suddenly the current cowards among the Islamic State will have to choose behind staying hidden and forgotten or getting more involved in the terror.
In Germany, the most influential public Wahhabi preacher is already considered too moderate for the radicals in the internet (Which is funny if you know which interpretation of the Quoran he cites, he is always at the most radical edge of it. A convert always has to go one step further to prove his true faith). Which is easy for those hidden in the Internet, since they never plan to be judged by their own standards. The Islamic State even went so far to call for his death as a traitor in their main propaganda magazine.
I am sorry, if this argumentation lacks some golden thread, I tried to structure it as good as I can, but it reads a bit confusing even to me. Would maybe be better suited as a blog post and could take way more literary work than the few minutes of a forum post.
