What is the Shia-Sunni divide?

Here is the history behind the Shia-Sunni divide

The Islamic State has claimed responsibility for two attacks that claimed at least 12 lives in Iran on Wednesday. With this, the flaring tensions between Sunnis and Shias are once again in the news.

Iran is a Shia Muslim majority state often in tension with Sunni states and extremist groups like the Islamic State or al-Qaeda. These attacks are the latest chapter in the story of the centuries-long strained relationship between Sunnis and Shias.

As a scholar of Islam and a public educator, I often field questions about Sunnis, Shias and the sects of Islam. What