since 1144, Pope Eugenius had recognised it the following year. While the city was usually happy to acknowledge the feudal lordship of the Pope, it was—even compared to other Italian city states—both "unusually self-aware, and also unusually idiosyncratic" compared to others. The commune was hostile to the Papacy. The Papacy was weak in the city of Rome. The heretic, Arnold of Brescia, had ruled since 1146 and was popular. He also had the support of the Roman Commune. The popularity of Arnold directly translated into hostility towards the popes. Chichele Professor Chris Wickham describes the relationship between the Pope and the lords of his Patrimony as one in which, because "their lords did not by any means all look to Rome [they] had to be coaxed back or brought back by force". Papal politics was beset by problems at home and abroad. The election of Adrian IV as Pope, comments the papal scholar Ian S. Robinson—and, indeed, the elections of his immediate predecessors—"took place in the shadow of the communal revolution in Rome". From Eugenius, Adrian inherited what Walter Ullmann has called a "mutual assistance pact" with the Emperor, the Treaty of Constance, signed the year of Eugenius' death. For the popes, its most important aspect was the stipulation that the crowning of the next emperor was contingent on expelling Arnold of Brescia from Rome. It also assured each party of the other's support against both King William in Sicily and the Byzantine Empire when necessary. The treaty was confirmed by Adrian in January 1155. Eugenius was a believer in the Gregorian doctrine of Papal supremacy, stating that Christ "gave to St Peter the keys of the kingdom of heaven, the power of both the earthly and the heavenly empire". From the beginning of his reign, Barbarossa sought to present himself as the heir to a long, established line of Roman Emperors, and likewise that his empire was a continuation of theirs. The historian Anne A. Latowsky explains how this was the cause of tension in the European polity: Norwich argues that, by now, whatever the public statements of either Papal or Imperial party, they were mutually antagonistic, and had been for many years. Even before Adrian's pontificate, he says, no peace treaty was strong enough to unite them for long: "The days when it had been realistic to speak of the two swords of Christendom were gone—gone since Gregory