Otto campaigning in northern Germany, Berengar II attacked the March of Verona in 958, which Otto had stripped from his control under the 952 treaty, and besieged Count Adalbert Atto of Canossa there. Berengar II's forces also attacked the Papal States and the city of Rome under Pope John XII. In autumn 960, with Italy in political turmoil, the Pope sent word to Otto seeking his aid against Berengar II. Several other influential Italian leaders arrived at Otto's court with similar appeals, including the Archbishop of Milan, the bishops of Como and Novara, and Margrave Otbert of Milan. After the Pope agreed to crown him as Emperor, Otto assembled his army to march upon Italy. In preparation for his second Italian campaign and the imperial coronation, Otto planned his kingdom's future. At the Imperial Diet at Worms in May 961, Otto named his six-year-old son Otto II as heir apparent and co-ruler, and had him crowned at Aachen Cathedral on 26 May 961. Otto II was anointed by the Archbishops Bruno I of Cologne, William of Mainz, and Henry I of Trier. The King instituted a separate chancery to issue diplomas in his heir's name, and appointed his brother Bruno and illegitimate son William as Otto II's co-regents in Germany. Otto's army descended into northern Italy in August 961 through the Brenner Pass at Trento. The German king moved towards Pavia, the former Lombard capital of Italy, where he celebrated Christmas and assumed the title King of Italy for himself. Berengar II's armies retreated to their strongholds in order to avoid battle with Otto, allowing him to advance southward unopposed. Otto reached Rome on 31 January 962; three days later, he was crowned Emperor by Pope John XII at Old St. Peter's Basilica. The Pope also anointed Otto's wife Adelaide of Italy, who had accompanied Otto on his Italian campaign, as empress. With Otto's coronation as emperor, the Kingdom of Germany and the Kingdom of Italy were unified into a common realm, later called the Holy Roman Empire. Papal politics On 12 February 962, Emperor Otto I and Pope John XII called a synod in Rome to legalise their relationship. At the synod, Pope John XII approved Otto's long-desired Archdiocese of Magdeburg. The Emperor had planned the establishment of the archdiocese to commemorate his victory at the Battle of Lechfeld over the Hungarians and to further convert the local Slavs