> How do you think they are linked? I know ISIS rhetoric talks about re-establishing a caliphate, world domination etc, but do you think there is any material connection?
Pretty much this. ISIS' world-view is that Muslims (by "Muslim" I mean Wahhabi/fundamentalist Sunni, not those dirty Shias/Alawites/Kurds/etc...) should conquer the world, much as Mohammed did (or at least his corner of the world), and the Rashidun caliphate. I wouldn't say there's so much a material connection, however there's certainly a very direct philosophical connection.
Anyhow, ISIS is a symptom, the culprit is Wahhabism. Those who blame the west for the rise of ISIS and Al Qaeda are correct, partially, in that the west destroyed Arab nationalism and Baathism, which was the competing ideology.
https://en.wikipedia.org/wiki/Wahhabism
The house of Saud first expanded when they allied themselves with the founder of Wahhabism. That's when they expanded into the Saudi state. Wahhabism itself seeks to be a restoration of Islam, free from idolatry and whatever they deem to be non-Islamic influences (the Saudis even went so far as to destroy Mohammed's grave/shrine). And of course the Saudis export this with oil money. You can see Saudi and Gulf sponsored madrassas all over the 3rd world, there's one in my wife's tiny community in a tiny 3rd world country (I've got some stories from there!). The Wahhabi (jihadist) world-view is what the Saudis export. The one thing Sunni terrorist groups have in common is that they've all grown out of Wahhabi ideology.
The connection to Islamic imperialism is partly historic, partly demographic. Historic in the sense that they want to recreate the 'real' caliphate. Demographic in the sense that, where there was Muslim conquest, there are Muslims today, and they have been courted by Wahhabi ideology. This is why IS terrorists come from diverse places, but all belong to one ideology. You won't find Shias or Alawites supporting IS.
Also, I never really addressed the initial post, but leaving the middle east alone won't work. Wahhabism as an ideology needs to be destroyed. It can come from within (I would say an Islamic 'enlightenment', although that came and went), or it can come from outside (military action against IS and eventual action against the Saudis would convince people that Wahhabism is simply a path to destruction). But pretending if we 'leave it alone' it'll go away is silly, because the ideology itself is pervasive, cultural in some places, and conquest is integral to the ideology. The actual solution - there needs to be another Arab nationalist/secular movement. Much like the 'west' became the 'west' after the enlightenment, and especially after the French revolution (the ideas of Liberté, égalité, fraternité and the French philosophers of that era could be considered the backbone of modern secular western culture).
Finally, I thought about it, could have probably made a more insightful first post, but the point stands. Islamic terror is about expansion and ideology, not disenfranchisement, even if the latter indirectly led to an acceptance of the former.