Iirc, the original plan for the 9/11 attacks hoped for the west to respond brutally to muslims in general, leading to a general muslim uprising against the west. I can't find the source right now, but I think it was either a high-ranking Al Qaeda member or leaked documents that came out a few years ago.
Now Al Qaeda is not ISIS, but I do believe that their strategists aren't too unhappy about further confrontation and distrust between muslims & non-muslims in general.
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Btw @Ford Prefect @Post Ironic @Maddy @Osman concerning the topic of the sanity/insanity of those commiting such horribly acts in general, I can highly recommand you the movie "The radical evil" by Stephan Ruzowitzky. It concerns itself primarily with German soldiers in the German occupied territories during WW II, and deals with exactly those questions discussed here. I don't know if there's a version with English subs available, but if you find one, watch it. One of the most interesting documentaries I've ever seen.
http://www.independent.co.uk/arts-e...he-film-that-nobody-wants-to-see-9959093.html