When John XII died suddenly in May 964, the Romans elected Pope Benedict V as his successor. Upon hearing of the Romans’ actions, Otto mobilized new troops and marched on Rome. After laying siege to the city in June 964, Otto forced the Romans to accept his appointee Leo VIII as Pope and exiled Benedict V. Third Italian Military Expedition Otto returned to Germany in January 965, believing his affairs in Italy had been settled. On 20 May 965, the Emperor's long-serving lieutenant on the eastern front, Margrave Gero, died and left a vast march stretching from the Billung March in the north to the Duchy of Bohemia in the south. Otto divided this territory into five separate smaller marches, each ruled by a margrave: the Northern March under Dietrich of Haldensleben, the Eastern March under Odo I, the March of Meissen under Wigbert, the March of Merseburg under Günther, and the March of Zeitz under Wigger I. Peace in Italy, however, would not last long. Adalbert, the son of the deposed King Berengar II of Italy, rebelled against Otto's rule over the Kingdom of Italy. Otto dispatched Burchard III of Swabia, one of his closest advisors, to crush the rebellion. Burchard III met Adalbert at the Battle of the Po on 25 June 966, defeating the rebels and restoring Italy to Ottonian control. Pope Leo VIII died on 1 March 965, leaving the chair of St. Peter vacant. The Church elected, with Otto's approval, John XIII as new Pope in October 965. John XIII's arrogant behavior and foreign backing soon made him disliked among the local population. In December of the same year, he was taken into custody by the Roman people but was able to escape a few weeks later. Following the Pope's request for help, the Emperor prepared his army for a third expedition into Italy. In August 966 at Worms, Otto announced his arrangements for the government of Germany in his absence. Otto's illegitimate son Archbishop William of Mainz would serve as his regent over all of Germany, while Otto's trusted lieutenant, Margrave Hermann Billung, would be his personal administrator over the Duchy of Saxony. With preparations completed, Otto left his heir in William's custody and led his army into northern Italy via Strasbourg and Chur. Reign from Rome Upon Otto's arrival in Italy, John XIII was restored to his papal throne in mid-November 966 without