Gregory particularly wanted to capture Sicily from Frederick's control, especially because Frederick had been abusing his power over the church there. Gregory's military leader was John of Brienne, Frederick's father-in-law, who had no particular reason to love Frederick.
Upon hearing of the problems, and that Gregory had invaded Campania in southwestern Italy (Frederick's territory) Frederick abandoned the Holy Land and sailed back home, landing at Brindisi (Italy's "heel") on 10 June 1229. He had his Crusader army, of course, and some of the residents of Campania were opposed to the pope's attempt to expand the Papal States, so joined his cause.
Gregory's attempt to gain control over Sicily was rebuffed by the Muslim population in the western part of the island: they did not want the head of the Christian church having political control over them. Likewise, he could not turn Germany against Frederick; although he was born in Italy (his mother was Constance, Queen of Sicily), his father was Holy Roman Emperor Henry VI, so Frederick was considered German through and through.
The approach of Frederick's forces toward Campania caused the papal forces to start to lose their nerve. In August 1229, Gregory renewed his excommunication of Frederick, along with some of his chief supporters. He was also making promises left and right to various towns about offering them privileges, remission of sins, and political autonomy if they would simply join him and become part of the Papal States, but few were interested. Based on Frederick's treaty in the East with al-Kamil, Gregory claimed that Frederick was working in alliance with Muslims, hoping to erode his support.
For his part, Frederick was sending messages to the heads of Europe, explaining his cause and requesting their support. By the end of October, John of Brienne had retreated back to papal territory, and Frederick's troops had re-taken any of the papal-invaded towns.
Rather than take the opportunity and excuse to invade the Papal States, Frederick stopped his army's advance and requested that negotiations take place. Negotiations led to the Treaty of San Germano, signed on 23 July 1230. Gregory gave permission for Cardinal Giovanna Colonna to absolve Frederick's excommunication. The final treaty was brokered by Leopold VI, Duke of Austria, whose seal is illustrated above.
Contemporary sources, analyzing the War of the Keys, discussed it in terms of the "doctrine of the two swords": the material sword and the spiritual sword. Gregory's letter to England referred specifically to exercising temporal power. This use by the pope of the material sword, since he failed to achieve his aims with the spiritual sword, was criticized by many. There are even competing troubadour poems on the topic.
What I really want to look at next, however, is John of Brienne's role in the war. For that, we have to understand the marriage to Frederick of John's daughter, Isabella of Jerusalem, who became Queen of Jerusalem at the age of 13, was married at 13 to a man she had yet to meet, and was dead by 16.
