Why did Alfonso X of Castile-Leon-Galicia relentlessly pursue his claims to the imperial thrones of the Holy Roman Empire and the Iberian 'empire', despite the high costs and probability of failure? WINNER: 2021-22 AHGBI-Spanish Embassy Publication Prize This book examines how the concept of imperium shapes the structure and ideology of the Estoria de España, the first major history of Spain in the vernacular, written under Alfonso's patronage. Through a detailed analysis of its Roman section, it explores how Alfonso's scriptorium translated, adapted, and expanded sources to bolster his imperial claim. More than a chronicle, the Estoria served as propaganda, reinforcing Alfonso's legitimacy by challenging papal authority in imperial elections and appealing to both the Castilian-Leonese nobility-whose financial support was crucial-and other Iberian monarchs. Alfonso's imperial vision drew not only on the Imperium Hispanicum of his father, Fernando III, but also on his Staufen lineage through his mother, Beatriz of Swabia, whose ties to the Holy Roman Empire likely influenced the historiographical models of the Estoria. By blending Iberian and European traditions, Alfonso positioned Castile as heir to both the Roman and Hohenstaufen legacies, setting a new standard for Iberian historiography that endured for centuries.