War of the Keys

The War of the Keys (1228–1230) was the first military conflict between Frederick II, Holy Roman Emperor, and the Papacy. Fighting took place in central and southern Italy. The Papacy made strong gains at first, securing the Papal States and invading the Kingdom of Sicily, while Frederick was away on the Sixth Crusade. Upon his return, he defeated the papal forces, forcing Pope Gregory IX to begin peace talks. After drawn-out negotiations, the treaty of San Germano terminated the conflict with no territorial changes.

The causes of the conflict lay in conflicting papal and imperial claims in central Italy, Frederick's failure to fulfill his agreement to lead a crusade on schedule and his supposed mistreatment of the Sicilian church. The emperor was excommunicated before he left on his crusade in June 1228. His representatives, with or without his permission, entered territory claimed by the Papacy and Gregory responded with war. His aim was to take Sicily, which was a fief of the church, back from Frederick.

Gregory raised troops and funds internationally, from as far afield as Portugal and Sweden. He sent two armies under the command of John of Brienne into the disputed territory in late 1228 and another army into the Kingdom of Sicily in January 1229. The war was going in his favour as late as May, but he was running low on funds and troops. The strategy of denying the ports to Frederick failed. The return of the emperor in June quashed the rumours spread by papal agents that he was dead and caused a rapid reversal of fortunes. The sieges of Sulmona and Capua were lifted by September and by October most of the kingdom had been recovered. Negotiations were opened in November and the active phase of the war ended.