Between the years 1474-8 Lorenzo had managed to incur the jealous hatred of Pope Sixtus IV, who was determined to become the greatest power in Christendom. This Pontiff skilfully detached Naples from her alliance with Florence and Milan by promising to be content with a nominal tribute of two white horses every year instead of the handsome annual sum she had usually exacted from this vassal. He congratulated himself especially on this stroke of policy, because he believed Venice to be too selfish as a commercial State {37} to combine with her Italian neighbours and so form another Triple Alliance. He then proceeded to win over the Duke of Urbino, who had been the leader of the Florentine army. He also thwarted the ambition of Florentine trade by purchasing the tower of Imola from Milan. The Medici, coveting the bargain for their traffic with the East, were too indignant to advance the money which, as bankers to the Papacy, they should have supplied. They preferred to see their rivals, the great Roman banking-house of the Pazzi, accommodating the Pope, even though this might mean a fatal blow to their supremacy. Lorenzo's hopes of a strong coalition against his foe were destroyed by the assassination of Sforza of Milan in 1474. The Duke was murdered in the church of St Stephen by three young nobles who had personal injuries to avenge and were also inspired by an ardent desire for republican liberty. The Pope exclaimed, when he heard the news, that the peace of Italy was banished by this act of lawlessness. Lorenzo, disapproving of all outbreaks against tyranny, promised to support the widowed Duchess of Milan. The control he exercised during her brief regime came to an end in 1479 with the usurpation of Ludovico, her Moorish brother-in-law. Then Riario, the Pope's nephew, saw that the time was ripe for a conspiracy against the Medici which might deprive them of their power in Italy. He allied himself closely with Francesco dei Pazzi, who was anxious for the aggrandisement of his own family. His name had long been famous in Florence, every good citizen watching the ancient _Carro dei Pazzi_ which was borne in procession at Easter-tide. The car was stored with fireworks set alight by means {38} of the Colombina (Dove) bringing a spark struck from a stone fragment of Christ's tomb. The citizens could not forget the origin of the sacred flame, for they had all heard in youth the story of the return of a crusading member of the Pazzi house with that precious relic. The two conspirators hoped to bring a foreign army against Florence and, therefore, gained the aid of Salviati, Archbishop of Pisa. The Pope bade them do as they wished, "provided that there be no killing." In reality, he was aware that a plot to assassinate both Lorenzo dei Medici and his brother, Giuliano, was on foot, but considered that it would degrade his holy office if he spoke of it.