On 27 January 1186 the German king Henry VI, son and heir of the Roman Emperor Frederick I ‘Barbarossa’, married Constance of Hauteville, heir to the throne of Sicily, in the Basilica of St Ambrose in Milan. The royal wedding sealed the union of the Kingdom of Sicily and the Holy Roman Empire, creating an enormous empire stretching from the shores of the North Sea to the beaches of Africa. The union also incited a major geopolitical conflict dominating European politics in the thirteenth century since it seriously compromised the sovereignty which the Roman papacy professed to exercise over all Christendom as well as the territorial integrity of the Papal State. Consequently, succeeding popes ( Innocent III, Gregory IX and Innocent IV) endeavoured to undo that union at all costs. The ensuing struggle between the Roman papacy and the Hohenstaufen emperors culminated in the deposition of the Emperor Frederick II by Pope Innocent IV on the First Council of Lyon in 1245, resulting in the final dissolution of the union of the Empire and the Kingdom of Sicily and the extermination of the Hohenstaufen race. By inviting a foreign prince, Charles of Anjou, brother to King Louis IX of France, to fight the last of the Hohenstaufens, papal politics ultimately turned the Italian peninsula into a battlefield for the two major powers of early-modern Europe: Spain and France.

The origins, the vicissitudes, and the consequences of the union of the Empire and the Kingdom of Sicily are the subject of the first part of this book. The second part deals with the trial of Frederick II at Lyon, the court and its competence, the law involved and, lastly, the execution and aftermath of the sentence of the court.