He made a mortal enemy of Pope Gregory IX (r. 1227–41), an imperious and querulous individual cut from much the same cloth as Innocent III, whose driving mission was to stamp out heresy, persecute unbelievers everywhere, and make all earthly princes aware that their power was nothing when compared with the papal majesty. Terrified that Frederick’s power in Sicily, southern Italy, Germany, and Lombardy would enable the Hohenstaufen dynasty to surround and dominate popes in the Papal States, Gregory repeatedly accused Frederick of heresy and encouraged other rulers to invade Hohenstaufen lands.
