He then turned to strengthening his Sicilian domains in preparation for the inevitable conflict with the Lombard League. Among his achievements in Sicily were his Liber Augustalis (1231), a new body of laws that were the most constructive of the era. In Germany, Frederick attempted to insure support for his Italian policy by granting the princes practically absolute authority within their territories. This policy led to a conflict with his son Henry, who objected to Frederick's virtual renunciation of his imperial rights in Germany. In 1234 Henry rebelled with the aid of the German towns, but Frederick easily deposed and imprisoned (1235) his son. At the Diet of Mainz (1235), Frederick issued a land peace establishing an imperial court of justice to try all cases except those involving the great vassals. This land peace is one of the monuments of imperial legislation.
In 1236 Frederick began a successful campaign against the Lombard cities, but in Mar., 1239, Pope Gregory IX joined the Lombards and excommunicated the emperor. Frederick issued a circular against the pope and seized most of the Papal States; in May, 1241, he captured a number of prelates en route from Genoa to a general council in Rome, and he was threatening Rome when Gregory died. While emperor and pope were thus at swords' points, Europe was threatened (1241) by a Mongol invasion under Batu Khan. The Mongols withdrew in 1242.
After the election (1243) of Pope Innocent IV, Frederick offered sweeping concessions to the pope and his allies, but the pope fled (1244) to Lyons, deposed Frederick at the Council of Lyons (1245), and gave the emperor's foes the privileges of Crusaders. The election (1246) of an antiking to Conrad IV, Frederick's younger son, plunged Germany into civil war. The war in Italy turned in Frederick's favor in 1250, but in December he died of dysentery.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.