CLASHES between Islam's two big sects, the Sunni and the Shia, take place across the Muslim world. In the Middle East a potent mix of religion and politics has sharpened the divide between Iran’s Shia government and the Gulf states, which have Sunni governments. A report by the Pew Research Centre, a think-tank, found that 40% of Sunnis do not consider Shia to be proper Muslims. So what exactly divides Sunni and Shia Islam, and how deep does the rift go?
