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The importance of the caliphate to the wider Muslim world was not in terms of religion, but instead of identity. The authority of Istanbul had been weakening for some time, not only in politics but also in its ability to provide religious leadership for hundreds of millions of Muslims across the globe. Other competing ideas about Islam were coming, meanwhile, from Iran and Saudi Arabia. But the caliphate was more important as a unifying institution, indeed as an idea. Since the coming of Islam in the 7th century, the caliphate in its various forms had provided a framework for the unity and identity of Muslims, an institution for the expression of the faith and, most importantly, a basis for political authority. It was that last part that turned out to be vital, so that its removal has...