Around 800 ce, both Western Christendom and Islam were politically united under imperial structures, the Carolingian Empire and the Abbasid Caliphate, respectively.

By the early tenth century, that unity was gone, yet the ideals of empire and cal­iphate remained, and the political history of the post-imperial Catholic and Islamic commonwealths in the five succeeding centuries cannot be written without appro­priate emphasis on these ideals. To understand why this is so, we must turn to a fa­mous episode of the first crusade.

More on the topic Around 800 ce, both Western Christendom and Islam were politically united under imperial structures, the Carolingian Empire and the Abbasid Caliphate, respectively.:

1. The Empire United
2. Politically Motivated Violence in the Nibelungenlied
3. Crossing the Boundaries between Christendom and Islam, 900–1050
4. The Abbasid Age
5. Bang Peter F., Bayly C.A., Scheidel Walter (eds.). The Oxford World History of Empire. Volume Two: The History of Empires. Oxford University Press,2020. — 1352 p., 2020