This work investigates the strategic and theological justifications behind the papacy's authorization of crusades against Christian lay powers in Italy between 1254 and 1343. Norman Housley, a prominent historian of the crusades, utilizes extensive archival research and primary source documentation to analyze the political mechanics of the Papal-Angevin alliance. He argues that these campaigns were not mere anomalies but were deeply integrated into the broader ecclesiastical and geopolitical objectives of the medieval church. The text provides a rigorous examination of how the crusading ideal was adapted to serve secular political ends within Western Europe.

Historians and scholars of the medieval period frequently cite this monograph as a foundational text for understanding the evolution of the crusading movement beyond the Levant. Readers often note the academic density of the prose and the meticulous attention to administrative detail provided by the author.