top of his strength, besieged Milan. He rejected all Milanese peace overtures, insisting on unconditional surrender. Milan and five other cities, however, held out, and, in October 1238, he had to raise the siege of Brescia. The emperor then went on to invade the Papal States, and was excommunicated by Pope Gregory IX. References Sources 1237 in Europe 1230s in the Holy Roman Empire 13th century in Italy Cortenuova 1237 Wars of the Guelphs and Ghibellines Conflicts in 1237 Battles in Lombardy Frederick II, Holy Roman Emperor