Ashura in New York. Great pictures, plus a very short history of the Shia/Sunni split from the Shia perspective:
It all started hours after Mohammad's death: while his son-in-law (and first cousin) Ali was attending to Mohammad's burial, others were holding a little election to see who should succeed Mohammad as the chief of what was by now an Islamic state. (Remember that by the end of his life, Mohammad was not only a religious leader, but the head-of-state of a significant polity.) The person soon elected to the position of caliph, or head-of-state, was an old companion of the prophet's named Abu Bakr. This was a controversial choice, as many felt that Mohammad had clearly indicated Ali as his successor, and after Abu Bakr took power, these people had no choice but to say that while he may have become the temporal leader of the young Islamic state, they did not recognize him as their divinely guided religious leader. Instead, Ali remained their spiritual leader, and these were the ones who would eventually come to be known as the Shia. The ones who elected Abu Bakr would come to be known as Sunni.
This is the Shia/Sunni split which endures to this day, based on this early disagreement.
This is interesting for how much of the conventional Sunni narrative it leaves out, I think (you can see my summary here, if you skip down to the paragraph beginning “Following the death of Muhammad”; I didn’t know that I was summarizing Sunni-only accounts at the time that I wrote it). In the Sunni version, the split doesn’t date from the time of Abu Bakr, but from after the rule of all four “rightly-guided” Caliphs (Abu Bakr, Umar, Uthman, then finally Ali), and after the civil war over Ali’s succession. Then, finally, when the community accepts the rule of the Umayyads for sake of peace, this is when the Sunni accept the split between temporal and spiritual leadership the Umayyads represent, and the Shia do not. Presumably from the Sunni point of view, the problem doesn’t arise during the rule of the first four Caliphs because they believe those rulers were also legitimate spiritual leaders. A Sunni recounting of the tale will also never fail to mention that Abu Bakr was Mohammad’s father-in-law, and (possibly only according to Sunni sources?) the first convert to Islam after the Prophet’s wife, Khadija. I don’t think I’ve ever seen a Sunni source mention that Ali was excluded from the shura council, or even note in passing that Shia Muslims complain that he was.
This kind of makes me wonder just how much the two groups differ in other areas of Muslim historiography. Shiism is pretty invisible in most mainstream scholarship on Islam, which rarely even bothers to note that Shia Muslims have radically different interpretations of some topics. Tmatt over at GetReligion frequently complains about the lack of attention journalists pay to explaining the differences between the two sects, but I have to say I’ve never come across a really good, comprehensive summary; I think I've probably only encountered a small slice of it in the course of reading other things.