Whalen shows how the 'Papal Revolution' of the late eleventh and twelfth century spurred churchmen to imagine a new world of Catholic unity under papal guidance. This new world would aim to unify all Christians, most especially eastern Christians, under the Supreme Pontiff and to bring Jews, Muslims, and pagans under his wing as converts to Christianity. The book takes us through a dazzling array of thinkers, always putting their thoughts in political and military context. Whalen's readings of more obscure authors are always enlightening, and his writing is lucid. This book will enjoy a very wide readership.