As a part of the settlement reached between him and Honorius III at the time of his coronation in 1220, Frederick had arranged to have his son Henry crowned as king of the Romans (i.e., ruler of Germany) while retaining the imperial, Italian, and Sicilian crowns for himself. Henry, encouraged by some people at his court, embarked on a policy that threatened Frederick’s relations with the German aristocracy. As a result, Frederick moved against Henry and placated the German nobles with his Constitution in Favour of the Princes (1232). Yet, whereas Frederick was willing to trade away some of his authority in Germany, he was determined to assert imperial rights in northern Italy. The Lombard cities, as early as 1226, had renewed the Lombard League. While Frederick was dealing with the problems caused by Henry’s rebellion between 1233 and 1235, the Lombards grew increasingly restless. Frederick confirmed their fears with his decision to summon a diet to Piacenza in 1236 to impose his imperial authority on them. His action demonstrated his lack of interest in papal efforts to arrange compromises between him and the Lombards. Moreover, the emergence of his chancellor, Pietro della Vigna, as his chief spokesman signaled a shift away from the quiet diplomacy between emperor and papacy that he had carried on with the aid of Hermann von Salza, the grand master of the Teutonic Order, a man respected by the pope and the Roman Curia. Pietro’s rhetoric was well fashioned for a propaganda war. On his side, Gregory appointed the strongly anti-imperial Cardinal James of Palestrina as his new legate in northern Italy and blocked Frederick’s planned diet. In his propaganda Frederick portrayed himself as the champion of orthodoxy working to prevent the spread of heresy in Lombardy, thus building on the theme of the cooperation between him and Honorius III aimed at stopping the growth of heresy. Gregory’s rhetoric appealed to papal claims based on the Donation of Constantine and expressed his earlier concerns about Frederick’s abuse of ecclesiastical rights.

The growing rift between Frederick and the papacy was not merely a revival of the papal-imperial conflict of the 12th century, though it certainly had elements in common. It had its immediate roots in the failure of the policy of cooperation employed under Innocent III, Honorius, and even Gregory himself. There is every indication that Frederick valued this relationship, but he increasingly came to see it as an obstacle to securing his imperial rights in northern Italy. The papacy had also worked to preserve good relations. But fear of Frederick’s policies in northern Italy evoked memories of Frederick Barbarossa among members of the Curia. Above all, neither the emperor nor the pope could turn back the clock on the development of the communes. In fact, from the very outset Frederick seemed more a pawn of the emerging forces in northern Italy than a restorer of the ideal of empire. The new forces were represented above all by two tyrants, Ezzelino and his brother, Alberigo, from the ancient da Romano family, who were working to expand their lordship from their base in Verona at the expense of towns such as Padua, Vicenza, and Brescia. Frederick relied on them for support, and in doing so he provoked the opposition of earlier supporters, such as Azzo, marchese d’Este, who now sided with the Lombards. Potentates such as the Romanos were the potential beneficiaries of Frederick’s military activities, more so than the emperor himself.

Buoyed by early success in northern Italy, Frederick returned to Germany. He even hoped to repair his differences with Gregory, who proved amenable. However, the attempted settlement broke down. On November 27, 1237, Frederick, back in Italy, dealt the Lombards a heavy blow in the Battle of Cortenuova. He followed his military success with a strong propaganda attack, chiefly directed against Gregory IX. But the victory won at Cortenuova proved difficult to convert into permanent gains. Milan continued to hold out. In the following summer Frederick laid siege to Brescia but failed to take the city. Gregory excommunicated the emperor, repeating previous papal criticisms. What was at stake, however, was not some ideological high ground but recognition that Frederick had violated the rights of the church in the kingdom of Sicily. Gregory attempted to use the machinery developed for the Crusade to the East to gather money and manpower to oppose Frederick, who, in turn, warned his fellow rulers of the danger that these efforts posed. The papacy accused Frederick of failing to support the Crusade mounted by Thibaut of Champagne in 1239 and delaying its departure for the East. Gregory wished to recall him to the program on which the papacy had been insisting since the reign of Innocent III, but Frederick’s own concerns were with his European domains. It was not that he opposed the papacy’s desire for a Crusade; he wanted to settle matters in Italy first.