How dangerous is the Sunni-Shia schism?

In 2014, with the Middle East convulsed by the murderous, self-styled Islamic State, a Daily Mail reader wrote a letter to the editor which began: ‘Are you confused by what is going on in the Middle East? Let me explain…’ Aubrey Bailey went on to describe the dizzying complexity of diplomatic relationships thrown into turmoil:

So, some of our friends support our enemies and some of our enemies are our friends, and some of our enemies are fighting our other enemies, whom we don’t want to lose, but we don’t want our enemies who are fighting our enemies to win… And all this was started by us invading a country to drive out terrorists who weren’t actually there until we went in to drive them out – do you understand now?

Barnaby Rogerson, a publisher, writer and lifelong traveller in the Islamic world, is an accomplished guide through this confusing terrain. He is the author of a biography of the Prophet Mohammed and a study of the roots of the Sunni-Shia schism, a defining divide to which he returns in The House Divided.