Divisions and Unities Between the Muslim Sects
While on the subject of Ashura, and on the malleability of doctrine, I thought I would provide an interesting example of how doctrinal differences can often be used to unite communities as well as divide them.
To provide the necessary context once again, Ashura is the day on which the Prophet Muhammad's grandson, Husayn bin Ali, was killed by the Umayyad Caliph Yazeed. The event is central to Shi'i theology as the paradigmatic case of how even the political leadership of the Muslim community can betray its own message and commit heinous and barbaric acts against Muhammad's rightful political, religious and material heirs. (There's much more to it than that, this is simplistic and reductive, but it's good enough for now.)
Now there is nothing in Sunni theology that would prevent Sunnis from accepting the fact that the killing of Husayn bin Ali was a bad thing for the caliph to do. Sunnis give an honored place to the first four caliphs following Muhammad, the fourth of which was Husayn's father, but after that, the situation changes and certainly the idea that any particular caliph was good or bad is relatively noncontroversial. (Some have argued that each and every caliph was deserving of his title and worthy of respect, but this is fringe to my mind and a little difficult to maintain with any level of credibility upon even cursory investigation.) Add to that the various statements of Muhammad indicating love for his grandson, and you can come up with a version of events which, while not as cataclysmic in the Sunni narrative as it would be in the Shi'i, largely corresponds.
One way in which this is expressed in the Sunni tradition is based on Prophetic statements or traditions (in the Sunni version of things) to fast on the tenth of Muharram for different reasons (more on that later). For the Shi'a, this can't be done, but still, we can use the doctrine to unite the two factions in a broader fashion than this. We all agree it was bad, some attach more importance to it, and some (the Sunnis) fast and others (the Shi'a) weep, but we all end up in the same place. That's how I remember learning it growing up. Why do they fast and we can't? It wasn't something important enough to really think about, any more than you would think about why we ate okra stew in the winter but not the summer. It just was what it was, nobody was good or bad in this.
But this very same doctrinal split can be approached from another way, that of the more divisive and sectarian. Why do Sunnis fast? Not, the more sectarian Shi'a will argue, because of the death of Husayn, but because the statements they attribute to Muhammad (disputed by the Shi'a) say that the Pharaoh drowned on this day, and the Jews were delivered from Egypt by the Prophet Moses. It is a celebration, in other words! Now it is technically true that this is the reason provided for the fast, one wouldn't expect in Sunni doctrine in particular that the Prophet would say one should fast for the future death of his grandson who was just a kid at the time, but for many Sunnis, it might come as a surprise that this is the basis, or, more likely, they sort of throw it all together. A bunch of stuff happened, some good, some bad, and fasting is solemn anyway and we do love Husayn bin Ali, so it's still a commemoration of a bunch of events, including the last one. But of course the more sectarian Sunni can say that Husayn bin Ali has nothing to do with this, it is a sin to innovate in Islam, so to commemorate his death is sinful and wrong, and we have to focus on Ashura as only respecting the former good events, not the latter.
And so notice what has happened in this latter version. The fasting has been transformed from commemroation to celebration, from respect to insult, and in the Shi'i version a fabrication driven by the Umayyads to distract people from the death of Husayn, in the Sunni version the only acceptable way to think of Ashura. One group has become the aiders and abettors of one of history's greatest theological wrongs and the other has become innovators, inventors, people who just add to doctrine after the message has already been completed, if not infidels then certainly trending in that direction. This is probably a fairly decent description of at least parts of Iraq today, though not all.
So yes there were dividers then, as there are unifiers now, but the trend is in the other direction. And yet the sources used to derive the conclusions are the same now as they were before, no changes there. The only real difference is what the people reading them seem to understand they say. Any my what a difference that makes.