It's a divide that dates back really to the early years of Islam fourteen hundred years ago. We can really trace it back to the death of the prophet Mohammed. Very broadly, Sunnis believe Abu Bakr, the father of Mohammed's wife Ayesha, was the rightful successor. Not all successors should be elected through consensus or all leaders – or caliphs as they're called.
Shias believe that Mohammed divinely ordained his cousin Ali to be the next Caliph, and that Ali and all other direct descendants of Mohammed and his successor are ultimately caliphs. Now this ultimately led to this split between Sunnis and Shias. There was a number of battles that were fought in the years following the death of the Prophet Mohammed as they tried to fight over the various successions, even though there were caliphs chosen. Ali himself was Caliph for five years before he was assassinated.
"Sunnis and Shias have lived together peacefully for centuries. They have prayed in the same mosques and some inter-marry."
But the history lesson aside, the Sunni-Shia divide in modern context is more about the regional rivalry between Iran and Saudi Arabia. That regional war is playing out as a proxy war in places like Syria and Yemen where both Iran and Saudi Arabia are both heavily involved. We also see it happen in places like Lebanon where both Saudi Arabia and Iran are influencing politics with some considerable success.
Although this split between Sunnis and Shias has proven to be a very bloody and geographically very wide, we do need to point out that Sunnis and Shias have lived together peacefully for centuries. They have prayed in the same mosques and some inter-marry. But it's really since the 1979 revolution in Iran, where Iran effectively became a theocracy, have we seen it turn into this destructive and destabilizing force within the Middle East.
I could give you a list of all the various struggles we've seen between both Sunnis and Shiites since that time and the role that dictators have played in suppressing Shia populations and Shia-dominated countries today. For example: Iraq is predominantly Shia but Saddam Hussain who was a Sunni, suppressed the Shia population for a very long time and also acted as a bulwark between Iran and other parts of the Middle East. We could say the same for Bahrain. We have a ruling family in Bahrain who are Sunni but the population is Shia. There's been a low level uprising since the start of the Arab Spring in 2011. The royal family there has cracked down on it very hard.
But the current context I’d argue the Sunni-Shia divide is really more a Saudi/Iranian split which has led to this general sense of divide across the region.