Then followed the memorable dispute, in 1472-1473, over the bishopric of Pisa, when the Pope's nominee, Francesco Salviati, was refused possession of his see, Pisa being one of the Tuscan towns under the control of Florence. To this Sixtus retaliated by seeking the friendship of Ferrante of Naples, a move Lorenzo anticipated by forming the league between Florence, Milan, and Venice. This league thoroughly alarmed both the Pope and Ferrante, and on the latter visiting Rome in 1475 a papal-Neapolitan alliance was formed. Even then hostilities might not have broken out had the young Duke of Milan not been assassinated in 1476, leaving an infant heir. This entailed a long minority, with all its dangers, and the apprehensions regarding these were not fanciful, inasmuch as Lodovico Sforza, uncle of the baby Duke, usurped the position under pretext of acting as regent. These crimes were plainly responsible for the Pazzi conspiracy in 1478 against the Medici themselves, a conspiracy which resulted in Giuliano, the younger brother of Lorenzo, being murdered in the cathedral, during mass, on the Sunday before Ascension, while Lorenzo himself was slightly wounded. That Sixtus and his nephew were accessories before the fact is now regarded as unquestionable. The vengeance taken by the enraged Florentines on the conspirators, their relatives, friends, and property, was terrible; the innocent, alas! being sacrificed indiscriminately with the guilty. The Archbishop of Pisa, Francesco Salviati, had entered eagerly into the scheme, and, although his sacred office prevented him from actually assisting in the deed, he was present in the cathedral until the signal was given for the perpetration of the deed, when he left the building to secure the Palazzo Publico. He was therefore summarily hanged with the others from the windows of the civic buildings. Sixtus made the execution, or the "murder" as he called it, of Salviati, his pretext for calling on his allies to make war on Florence. When he saw, however, that this action was only throwing the city more completely than ever into the arms of the Medici, he changed his tactics and said he had no quarrel with "his well-beloved children of Florence," but only with "that son of iniquity and child of perdition, Lorenzo de' Medici," and those who had aided and abetted him, among whom the humanists were expressly mentioned. Against Lorenzo and his associates a brief of excommunication was launched, and the city was urged to regain the papal favor by surrendering the offenders.