The death of Henry VI, Emperor of the Holy Roman Empire and King of Sicily, in 1197 brought the crown of Sicily to Henry's remarkable son Frederick II before his third birthday. By the age of 21, through clever diplomacy and military action, Frederick had attained undisputed control of his father's Imperial crown as well and controlled the largest realm of Europe. Later, as a successful, though somewhat reluctant, crusader in the Holy Land, he was crowned King of Jerusalem--a title he claimed through his second wife Iolande, a daughter of John, Count of Brienne.

Even within his own lifetime Frederick II was widely regarded as one of the most brilliant rulers in the history of European monarchy, combining in a unique mixture the cultural heritage of his German father and Sicilian mother. He was strongly influenced by Islamic, Hebrew and Christian scholars, all of whom he cultivated at his court in Sicily. Frederick II himself was fluent in six languages and a student of mathematics, philosophy, natural history, medicine and architecture. He was a poet as well, and one of his principal courtiers composed the first Italian sonnet. All these interests led to his being apothesized as Stupor Mundi--the "wonder of the world."

Frederick II's reign in his kingdom in Sicily and southern Italy was more successful than his experience as Emperor in the north. He moved powerfully to end the autonomy of the Sicilian feudal lords and developed a strong royal administrative organization.

In his Empire to the north, however, he faced a number of intractable obstacles. The princes of Germany were strongly independent, with the right even to elect the Emperor. Frederick II had no choice but to accept the reality of their power. At the same time, he faced two major challenges in northern Italy. The cities of the Lombardy region, strengthened by new economic prosperity, were restive and constantly testing the rule of the Empire. Then, after Frederick's son Enzio through marriage became King of Sardinia and added that island territory to the Empire, Pope Gregory IX became alarmed at the encirclement he perceived. Gregory was already at odds with Frederick II over the Pope's claim to supremacy in Sicily. In fact, Gregory had previously attempted unsuccessfully to enforce his claims by dispatching an invading army while Frederick was away in the Holy Land on Crusade. Now Gregory threw his support to the rebellious cities of Lombardy.

At first, military success ran to Frederick. Rome itself was threatened on several occasions. By the time of Frederick's death in 1250, the tide of affairs had turned. Later, with Frederick II's son Manfred on the throne of Sicily, Pope Clement IV found the key for a major victory.