The Papacy plotted continually to undermine the rule of Frederick II and his descendants as kings of Sicily. The object was to break the perceived encirclement of the Papal States by the Holy Roman Empire and to install a more pliable ruler who would acknowledge the overriding supremacy in the Kingdom of Sicily of the Pope himself. Pope Gregory IX's 1228 invasion was repelled by Frederick during his lifetime, but after Frederick's death Pope Clement IV took a more subtle and indirect approach to unseat Frederick's illegimate son Manfred. Manfred ruled the Kingdom of Sicily first as regent and protector for Frederick's son and grandson and then in his own name.

Clement induced Charles, Count of Anjou and brother of King Louis IX of France, to take up his cause. In 1265 Clement declared Charles to be new King of Sicily--but only as a feudal subject of the Pope, of course. At first the scheme seemed to work exactly as planned. Charles brought a powerful army and fleet from France to southern Italy, where he defeated and executed Manfred and began his rule as Charles I.

The Pope and Charles had failed, however, to give adequate weight to three factors: the arrogance that the Anjou French as an occupying army would display toward the Sicilians, the resentment that would be created as a result among the normally long-suffering Sicilians--and the effect of the earlier marriage of Manfred's daughter Constance to Peter III, King of the Spanish kingdom of Aragon. Sixteen years later, on Easter Sunday 1282, these explosive factors were to produce the bloody Sicilian Vespers.