After the death of Pope Nicholas II in July 1061, two different groups met to elect a new pope. The cardinals met under the direction of Hildebrand (who later became Pope Gregory VII) and elected Alexander II on September 30. Alexander had been one of the leaders of the reform party in his role as Anselm, Bishop of Lucca.
Twenty-eight days after Alexander's election an assembly of German and Lombard bishops and notables opposed to the reform movement was brought together at Basel by the Empress Agnes as regent for her son Henry IV, and was presided over by the Imperial Chancellor Wilbert. They elected on October 28, 1061, the bishop of Parma, Cadalus, who assumed the name of Honorius II.
With the support of the Empress and the nobles, in the spring of 1062 Cadalus with his troops marched towards Rome to claim the papal seat by force. Bishop Benzo of Alba helped his cause as imperial envoy to Rome, and Cadalus advanced as far as Sutri. On April 14 a brief but bloody conflict took place at Rome, in which the forces of Alexander lost and Cadalus got possession of the precincts of St. Peter's.
Duke Godfrey of Lorraine arrived in May of 1061, and induced both rivals to submit the matter to the king's decision. Cadalus withdrew to Parma and Alexander II returned to his see in Lucca, pending Godfrey's mediation with the German court and the advisers of the young German king, Henry IV.
In Germany, meanwhile, a revolution had taken place. Anno, the powerful Archbishop of Cologne, had seized the regency, and the Empress Agnes retired to the convent at Fructuaria in Piedmont. The chief authority in Germany passed to Anno, who was hostile to Cadalus.
Having declared himself against Cadalus, the new regent at the Council of Augsburg, (October, 1062), secured the appointment of an envoy to be sent to Rome for the purpose of investigating charges of simony against Alexander. The envoy, Burchard, Bishop of Halberstadt (Anno's nephew) found no objection to Alexander's election. Alexander II was recognized as the lawful pontiff, and his rival, Cadalus, excommunicated in 1063.
The antipope did not, however, abandon his claims. At a counter-synod held at Parma he defied the excommunication. He gathered an armed force and once more proceeded to Rome, where he established himself in the Castel Sant'Angelo.
The ensuing war between the rival popes lasted for about a year. Honorius eventually gave up, left Rome as a fugitive, and returned to Parma.
The Council of Mantua, on Pentecost, May 31, 1064, ended the schism by formally declaring Alexander II to be the legitimate successor of St. Peter. Cadalus, however, maintained his claim to the papal chair to the day of his death in 1072.