Razib put up an interesting post on this topic on his blog . I think his point is that no religion is a "religion of the book". People make the religion and they remake it as time demands. Messily and unpredictably in many cases, but still, it moves. And in this sense, Islam is no more fixed in stone by what is written or not written in it's text (or texts) than any other religion.
Someone then commented (and I urge you to read the post and the comments, and the hyperlinks, they are all relevant and make this post clearer) as follows:
"Well, if you take the Old Testament and Koran at face value, the OT is more violent. The interesting question is then why Islam ends up being more violent than Judaism or Christianity, and for that I agree you have to thank subsequent tradition and reinterpretation of the violence in the text. It appears that for whatever reason Islam has carried out less of this kind of reinterpretation, so what was originally a less violent founding text ends up causing more violence because it is being interpreted much more literally."
I replied there, and then thought I would put that reply up as a new post here because I want to see what people think of this quick and off-the-cuff comment. THEN, I can maybe improve it in a final new post this weekend. So, without further ado, my comment:
There is an easier explanation. Islam the religion we know today (classical Islam of the four Sunni schools and it's Shia counterparts) developed in the womb of the Arab empire. It is evident that it provided a unifying ideology and a theological justification for that empire (and in the case of various Shia sects, varying degrees of resistance or revolt against that empire), but at the very least, they grew and formed together; one was not the later product of the fully formed other. Being the religion of a (very successful and impressive) imperialist project, it's "official" mature Sunni version obviously has a military-supremacist feel to it.
Whether the text canonized as "foundational document" does or does not fully explain the imperialism and supremacism is a red herring. The Quran is a fairly long book, but to an outsider it should be immediately obvious that you can create MANY different Islams around that book and if you did it all over again, NONE of them have to look like classical Sunni Islam. The details of Sunni Islam (who gets to rule, what daily life is supposed to look like, how non-Muslims should be treated, etc) are not some sort of direct and unambiguous reading of the Quran. Even the 5 daily prayers are not specified in the Quran. The schools of classical Sunni Islam are supposedly based on the Quran and hadith, but the Quran and the hadiths are clearly cherry picked and manipulated (and in the case of the hadiths, frequently just invented) based on the perceived needs of the empire, the ulama, the individual commentators, human nature, economics, whatever (insert favorite element here).
So in principle, we should be able to make new Islams as needed (and some of us have indeed done so over the centuries...the Ismailis being one extreme example) and I am sure many of us will do that in the days to come as well. The Reza Aslan types are right about that (though i seriously doubt that HE can make anything lasting). In fact, in terms of practice, millions of Muslims have already "invented new Islams". Just as a random example, most contemporary Muslims do not have concubines and do not buy and sell slaves (and find the thought of doing so shocking). They take oaths of loyalty to all sorts of "un-Islamic" states and most of them turn out to be loyal at least to the same degree as their other fellow citizens of various hedonistic modern states. And so on and so forth.
What sets them apart is their inability (until now) to publicly and comfortably articulate a theological framework that rejects medieval (aka no longer fashionable) elements of classical Sunni Islam. And this is especially a problem in Muslim majority countries. What stops them? I think apostasy and blasphemy laws (and the broader memes that uphold those laws) play a big role. King Hussein or Benazir Bhutto or even Rouhani may have private thoughts about changing X or Y inconvenient parts, but to speak up would be to invite accusations of blasphemy and apostasy. So they fudge and do one thing while paying lip service to another. Unfortunately, this means the upholders of classical Islam (and ISIS and the Wahabis are not as far from the mainstream Sunni Ulama in theory as is sometimes portrayed, though clearly they are pretty far in practice) have the edge in debates in the public sphere. This IS a serious problem. But the internet has made it very hard to keep inconvenient thoughts out of view. So there will be much churning. Eventually, some countries will emerge out of it better than others.
ISIS itself will not. Of course, in principle, anything is possible. But we can still make predictions based on whatever model we have in our head. Like most predictions in social science and history, they will not be mathematical and precise and our confidence in them (or our ability to convince others, even when others accept most of our premises) will not be akin to the predictions of mathematics or physics. But for whatever it's worth, I don't think ISIS will settle into some semi-comfortable equilibrium. They will only destory and create chaos. And eventually they will be destroyed, though it is possible (maybe even likely) that large parts of Syria, Iraq and North Africa could become like Somalia. Too messy, too violent and too poor to be worth the effort of colonizing even by intact nearby states. But probably not forever. The real estate is too valuable and eventually someone will bring order to it. Probably using more force and cruder methods than liberal modern intellectuals are comfortable with.