Actually, my reason is exactly correct, and there's two good examples from history there that prove my point, namely where major Islamic polities ceased to exist because they engaged in orgies of intolerance and extremism. The first is Al-Andalus (Muslim Iberia) under the Almohads, who were much more intolerant of religious minorities (especially Jews) than their forebearers, and who vastly hastened the demise of Muslim Iberia during their rule. Its probable that Al-Andalus would have lasted longer - perhaps even to the present day consider that the last sultan of Granada was unseated in the same year as Columbus discovered the New World) - without this episode. The second example is the Mughal Empire under Aurangzeb, who ruined the Mughal Empire through his widespread progromes and military campaigns against Hindus - the end effect was that he left the entire subcontinent in a destitute condition to be taken over by the English.
As for the Sunni-Shia divide, I will agree with you in so far as that in the talk of right-wing nutjobs in the West, this is a concept that doesn't show up. Instead, the Islamic world is prefered to be viewed as a monolithic, homogenous blob on the map, so that you can justify to perceive it as a "threat" to Western civilization.
you know the history, probably even better than me
but you give the answer yourself, correct me if i'm wrong
indeed the example we always get of tolerant and openminded Muslims in history were the Morish Omayades in Spain, but they forget that they were replaced by the very strict Almohads
weren't the Almohads themselves replaced by the rather fundamentalistic Christian reconquistadores?
and you state yourself that the - contrary to his predecessors - very strict Aurangzeb left the country in chaos till the British came
i don't want to relive this all again in Europe
the fact that ISIS is able to recruit so easily so many of its warriors in Europe by some preachers with a radical vocabulary of hatred doesn't make me optimistic at all
and the fact that many of them are 'converts' and adventurers doesn't make it better, it's the vocabulary that does it
and yes, most non-Muslims don't know about the century-long history of divide and hatred between Sunni and Shia
but as I told, Muslims are embarrased by this and don't talk about it, neither do they want us to talk about it
it seems to me they themselves cannot explain this if Islam is realy 'the religion of love and peace'