Mohammed revealed his new faith in 610 and it was known as Islam or submission to God. He gathered followers quickly and by the time of his death in 632 had set the stage for the building of an empire. But the Sunni-Shia divide was the result of disagreement over future leadership. The disagreement was simple. The Shia believe that only the descendents of Mohammed could rule, and the Sunni believe that being part of Mohammed’s bloodline was not necessary. The Sunni were more powerful and have a long history of persecuting Shia.
There were further splits within the Shia (e.g. “the Twelvers”), the details of which are not of concern here, but the result is the modern-day distribution of majority Shia in Iran, Iraq, Azerbaijan, and Bahrain and about 40 countries are Sunni.
This modern ethnoreligious conflict
The current sectarian and political differences between the two are due in no small part to the Ayatollah Khomeini in Iran who instituted an Islamic government based on Shia religious principles. Organizations like Hamas and the Muslim Brotherhood are Sunni and do not accept such a version of Islam. Saudi Arabia, a Sunni country, recently sent troops into Yemen (a key strategic concern to the United States) to repel Iranian supported agitators as well as the Houthis. Yemen shares a border with Saudi Arabia. And some scholars have argued that the Sunni puritanical sect known as Wahhbism was in response to Shia Iran. The tensions in Yemen, the Iran-Iraq war in 1980-1988, and the organization of militants in Afghanistan to fight the Soviet Union are all the result of Sunni-Shia tensions. And you might recall that Saddam Hussein was a Sunni ruling over a majority of Shia. Iran supports Bashar al-Assad even though he is an Alawite and a member of the Shia minority sect.
Sunni and Shia governments constantly worry about their grip on power, especially in the wake of protest movements in places like Tunisia and Egypt. The Arab awakening has spread along the sectarian divide especially when minority sects are the ruling power. This is true in Bahrain where Shia are the majority but there is a Sunni ruling family, and of course the Alawite in Syria rule over a Sunni majority. The Civil War in Syria is a classic sectarian tension and a proxy war between Sunni and Shia powers.
These authoritarian regimes, especially where a minority religious group rules over a majority, rely on authoritarian governments closely aligned with their military to maintain the order. These authoritarian governments are sometimes preferred because they result in stability. Sometimes leading scholars even suggest that these cultures are not going to be receptive to American reforms especially with respect to democracy creation. Consequently, they argue for the desirability of authoritarian regimes as illiberal as they are. But the Arab Awakening must be explained. Surely cultural, technological, and economic factors can be a combustible mixture. The Sunni and the Shia provide the spark for this mixture and bubble underneath most political change in the Arab world.