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Second, Third and Fourth Italian Campaigns: 1158–1174

The retreat of Frederick in 1155 forced Pope Adrian IV to come to terms with King William I of Sicily, granting to William I territories that Frederick viewed as his dominion. This aggrieved Frederick, and he was further displeased when Papal legates chose to interpret a letter from Adrian to Frederick in a manner that seemed to imply that the imperial crown was a gift from the Papacy and that in fact the Empire itself was a fief of the Papacy. Disgusted with the pope, and still wishing to crush the Normans in the south of Italy, in June 1158, Frederick set out upon his second Italian expedition, accompanied by Henry the Lion and his Saxon troops. This expedition resulted in the revolt and capture of Milan, the Diet of Roncaglia that saw the establishment of imperial officers and ecclesiastical reforms in the cities of northern Italy, and the beginning of the long struggle with Pope Alexander III. Milan soon rebelled again and humiliated Empress Beatrice (see Legend below).

The death of Pope Adrian IV in 1159 led to the election of two rival popes, Alexander III and the antipope Victor IV, and both sought Frederick's support. Frederick, busy with the siege of Crema, appeared unsupportive of Alexander III, and after the sacking of Crema demanded that Alexander appear before the emperor at Pavia and to accept the imperial decree. Alexander refused, and Frederick recognised Victor IV as the legitimate pope in 1160. In response, Alexander III excommunicated both Frederick I and Victor IV. Frederick attempted to convoke a joint council with King Louis VII of France in 1162 to decide the issue of who should be pope. Louis neared the meeting site, but when he became aware that Frederick had stacked the votes for Victor, Louis decided not to attend the council. As a result, the issue was not resolved at that time.

The political result of the struggle with Pope Alexander was an alliance formed between the Norman state of Sicily and Pope Alexander III against Frederick. In the meantime, Frederick had to deal with another rebellion at Milan, in which the city surrendered on 6 March 1162; much of it was destroyed three weeks later on the emperor's orders. The fate of Milan led to the submission of Brescia, Placentia, and many other northern Italian cities. In August 1162 he triumphantly entered Turin and was crowned with his consort in the cathedral on August 15. Returning to Germany towards the close of 1162, Frederick prevented the escalation of conflicts between Henry the Lion from Saxony and a number of neighbouring princes who were growing weary of Henry's power, influence, and territorial gains. He also severely punished the citizens of Mainz for their rebellion against Archbishop Arnold. In Frederick's third visit to Italy in 1163, his plans for the conquest of Sicily were ruined by the formation of a powerful league against him, brought together mainly by opposition to imperial taxes.

In 1164, Frederick took what are believed to be the relics of the "Biblical Magi" (the Wise Men or Three Kings) from the Basilica di Sant'Eustorgio in Milan and gave them as a gift (or as loot) to the Archbishop of Cologne, Rainald of Dassel. The relics had great religious significance and could be counted upon to draw pilgrims from all over Christendom. Today they are kept in the Shrine of the Three Kings in the Cologne cathedral. After the death of the antipope Victor IV, Frederick supported antipope Paschal III, but he was soon driven from Rome, leading to the return of Pope Alexander III in 1165.

In the meantime Frederick was focused on restoring peace in the Rhineland, where he organized a magnificent celebration of the canonization of Charlemagne at Aachen, under the authority of the antipope Paschal III. Concerned over rumours that Alexander III was about to enter into an alliance with the Byzantine Emperor Manuel I, in October 1166 Frederick embarked on his fourth Italian campaign, hoping as well to secure the claim of Paschal III and the coronation of his wife Beatrice as Holy Roman Empress. This time, Henry the Lion refused to join Frederick on his Italian trip, tending instead to his own disputes with neighbors and his continuing expansion into Slavic territories in northeastern Germany. In 1167 Frederick began besieging Ancona, which had acknowledged the authority of Manuel I; at the same time, his forces achieved a great victory over the Romans at the Battle of Monte Porzio. Heartened by this victory, Frederick lifted the siege of Ancona and hurried to Rome, where he had his wife crowned empress and also received a second coronation from Paschal III. His campaign was halted by the sudden outbreak of an epidemic (malaria or the plague), which threatened to destroy the Imperial army and drove the emperor as a fugitive to Germany, where he remained for the ensuing six years. During this period, Frederick decided conflicting claims to various bishoprics, asserted imperial authority over Bohemia, Poland, and Hungary, initiated friendly relations with Manuel I, and tried to come to a better understanding with Henry II of England and Louis VII of France. Many Swabian counts, including his cousin the young Duke of Swabia, Frederick IV, died in 1167, so he was able to organize a new mighty territory in the Duchy of Swabia under his reign in this time. Consequently, his younger son Frederick V became the new Duke of Swabia in 1167, while his eldest son Henry was crowned King of the Romans in 1169, alongside his father who also retained the title.