a necessary preliminary to the enforcement of the imperial rights in Italy. Issuing a general order for peace, he made lavish concessions to the nobles. Abroad, Frederick intervened in the Danish civil war between Svend III and Valdemar I of Denmark and began negotiations with the Eastern Roman Emperor, Manuel I Comnenus. It was probably about this time that the king obtained papal assent for the annulment of his childless marriage with Adelheid of Vohburg, on the grounds of consanguinity (his great-great-grandfather was a brother of Adela's great-great-great-grandmother, making them fourth cousins, once removed). He then made a vain attempt to obtain a bride from the court of Constantinople. On his accession, Frederick had communicated the news of his election to Pope Eugene III, but had neglected to ask for papal confirmation. In March 1153, Frederick concluded the Treaty of Constance with the Pope, wherein he promised, in return for his coronation, to defend the papacy, to make no peace with king Roger II of Sicily or other enemies of the Church without the consent of Eugene, and to help Eugene regain control of the city of Rome. First Italian Campaign: 1154–55 Frederick undertook six expeditions into Italy. In the first, beginning in October 1154, his plan was to launch a campaign against the Normans under King William I of Sicily. He marched down and almost immediately encountered resistance to his authority. Obtaining the submission of Milan, he successfully besieged Tortona on 13 February 1155, razing it to the ground on 18 April. He moved on to Pavia, where he according to some historians he received the Iron Crown and the title of King of Italy on 24 April in the Basilica of San Michele Maggiore. Others historians instead suggest his coronation took place in Monza on 15 April. Moving through Bologna and Tuscany, he was soon approaching the city of Rome. There, Pope Adrian IV was struggling with the forces of the republican city commune led by Arnold of Brescia, a student of Abelard. As a sign of good faith, Frederick dismissed the ambassadors from the revived Roman Senate, and Imperial forces suppressed the republicans. Arnold was captured and hanged for treason and rebellion. Despite his unorthodox teaching concerning theology, Arnold was not charged with heresy. As Frederick approached the gates of Rome, the Pope advanced to meet him. At the royal tent the king received him, and after