As the religion spread and the Abbasid dynasty declined, the civilization of Islam, unlike that of China but similar to Western Christendom, operated without a dominant political center, bound more by a shared religious culture than by a shared state. Twice that civilization was threatened from outside. The most serious intrusion came during the thirteenth century from the Mongols, whose conquest of Central Asia and Persia proved devastating while incorporating many Muslims within the huge Mongol domains (see Chapter 11). Less serious but more well known, at least in the West, were the Christian Crusaders who established in the twelfth and thirteenth centuries several small and temporary outposts along the eastern Mediterranean (see Chapter 10).

Despite these external threats and its various internal conflicts, Islamic civilization flourished and often prospered, embracing at least parts of virtually every other civilization in the Afro-Eurasian hemisphere. It was in that sense “history’s first truly global civilization,” although the Americas, of course, were not involved.28 What held this Islamic world together? What enabled many people to feel themselves part of a single civilization despite its political fragmentation, religious controversies, and cultural and regional diversity?