This study argues that contemporary historical works are an unparalleled source for charting the neglected subject of the implementation and impact in northern Italy of the crusade that was launched against Frederick II in 1240; and that a mostly uncritical acceptance of that crusade became a topos in works by laymen as well as clerics across the region. Above all, those works reveal that, while pro-papal factions are a fixture of scholarship on the Italian cities during the central and late

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... dle Ages, adherence to the Church actually became an explicit and distinguishing feature of Lombard factions only when the crusade was launched against Frederick II.