Otto I or Otto the Great, 912–73, Holy Roman emperor (962–73) and German king (936–73), son and successor of Henry I of Germany. He is often regarded as the founder of the Holy Roman Empire. Boldly developing the policies that his father had begun, Otto brought the Middle Kingdom of the Carolingian Lothair I (see Verdun, Treaty of), including Italy, Burgundy, and Lotharingia, under German influence and broke the independence of the duchies. The rebellions of Otto's brother, Henry, and of Duke Eberhard of Franconia were ended by the battle of Andernach (939) and Henry's submission (941). King Louis IV of France, hoping to gain Lotharingia, had assisted the rebels, and Otto campaigned against him (940) with Hugh the Great; in 942, however, Otto and Louis reached an agreement, and Otto helped Louis to defeat Hugh (950). In 951, Otto invaded Italy, taking advantage of an appeal from the widowed Italian queen, Adelaide, who was about to be forced into a marriage with the son of Berengar II. Defeating Berengar, Otto assumed the title king of the Lombards, married Adelaide, and returned to Germany, where Berengar eventually paid him homage. In Germany another revolt was brewing. Rivalry and jealousy among the dukes, particularly against Otto's brother, Henry, whom he had made duke of Bavaria in 947, resulted in a rebellion in 953 led by Conrad the Red and Otto's son Duke Ludolf of Swabia. New attacks by the Magyars ended the rebellion and forced the dukes to form a united front against the invaders, who were defeated (955) in the Lechfeld. Otto had already begun to counter the ducal power by creating the "Ottonian system," entailing close alliance between the crown and the higher prelates. An important exponent of the alliance was his brother and chief adviser, St. Bruno, archbishop of Cologne, whom Otto made duke of Lotharingia. Meanwhile, in Italy, Berengar II resumed his aggression. Pope John XII appealed to Otto, who entered Rome and was crowned emperor early in 962, reviving the imperial title of the Carolingians and legitimizing the German kings' claim to the Middle Kingdom; Otto thus linked the destinies of Italy and Germany. John soon found the emperor too powerful and, while Otto was campaigning against Berengar, secretly negotiated with Otto's enemies. Otto hastened back to Rome (963), deposed John, and installed a new pope, Leo VIII. The Romans, seeing all independence lost, rose in 964 and restored John, but John died the same year and Otto reinstated Leo. Otto's campaign (966–72) to gain control over S Italy was unsuccessful, but a minor diplomatic triumph was scored in 972 when Emperor John I of Byzantium gave a Greek princess in marriage to Otto's son and successor, Otto II.