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Why did Frederick I refuse to support a Byzantine attack on the Norman kingdom of Sicily?

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Medieval manuscript miniature showing Frederick I as a crowned crusader holding an orb and scepter, with Henry of Schaftlarn kneeling and presenting a book to him. — Frederick I Frederick I as a Crusader with (right) Henry of Schäftlarn dedicating to him a copy of the *History of the First Crusade* by Robert of St. Remy; miniature from a manuscript in the Vatican Library.

Frederick I refused to support a Byzantine attack on the Norman kingdom of Sicily primarily due to his political strategy regarding the papacy and his desire to assert the independence of the Holy Roman Empire from Byzantine influence.1

When he ascended to the throne, Frederick sought to establish the empire's predominance over European monarchies. To achieve this, he rejected the existing alliance between his predecessor, Conrad III, and the Byzantine emperor Manuel I Comnenus against the Normans. By distancing himself from Byzantium, Frederick pressured Pope Eugenius III into signing the Treaty of Constance in 1153. Under this treaty, Frederick explicitly promised not to concede any Italian land to the Byzantine emperor, effectively blocking Byzantine territorial ambitions in Italy.1

Furthermore, Frederick viewed the alliance with the "kingdom of the Greeks" as an unequal partnership that threatened his imperial authority. Consequently, even when Manuel I offered him a Byzantine princess in marriage and attempted to induce him to fight the Normans, Frederick refused, prioritizing his own political autonomy and the consolidation of his power in Italy over Byzantine interests.12