A man of extraordinary culture, energy, and ability – called by a contemporary chronicler stupor mundi (the wonder of the world), by Nietzsche the first European, and by many historians the first modern ruler – Frederick established in Sicily and southern Italy something very much like a modern, centrally governed kingdom with an efficient bureaucracy.
20th century treatments of Frederick vary from the sober (Wolfgang Stürner) to the dramatic (Ernst Kantorowicz). However, all agree on Frederick II's significance as Holy Roman Emperor. In the judgment of British Historian Geoffrey Barraclough, Frederick's extensive concessions to German princes—which he made in the hopes of securing his base for his Italian projects—undid the political power of his predecessors and postponed German unity for centuries.
However, the modern approach to Frederick II tends to be focused on the continuity between Frederick and his predecessors as Kings of Sicily and Holy Roman Emperors, and the similarities between him and other thirteenth-century monarchs. David Abulafia, in a biography subtitled "A Medieval Emperor," argues that Frederick’s reputation as an enlightened figure ahead of his time is undeserved, and that Frederick was mostly a conventionally Christian monarch who sought to rule in a conventional medieval manner.