Maximilian I. Alexander VI allied himself with Louis XII in return for the King's support in establishing Alexander's son Cesare Borgia, the Duke of Valentinois, as ruler in Romagna. Alexander issued a papal bull on 9 March 1499 to invalidate the investiture of the feudal lords, including Caterina. When the French army left Milan with Cesare to begin the conquest of Romagna, Ludovico il Moro regained the Duchy with the help of the Austrians. Caterina sought relief from Florence against the approaching French army, but Florence was threatened by the Pope, so she was left alone to defend herself. She immediately began to recruit and train many soldiers and began to store weapons, ammunition and food. She reinforced the defenses, especially that of Ravaldino where she resided and which was already considered impenetrable. She also sent her children to Florence. On 24 November, Cesare Borgia arrived in Imola. The city gates were opened by the inhabitants, and he was able to take possession, after having conquered the fortress where the castellan Dionigi Naldi of Brisighella had resisted for several days. After seeing what had happened there, Caterina asked the people of Forlì if they also wanted to capitulate to Borgia, or if they wanted to be defended and endure the resulting siege. Because the people hesitated in answering, Caterina absolved the citizens of Forlì of their oath of fealty, and sealed herself in Ravaldino. On 19 December, the Duke of Valentinois took possession of Forlì and began the siege of the fortress. Caterina repeatedly refused all offers of peace, from Cesare and from Cardinal Riario. In response, Cesare offered 10,000 ducats for her, dead or alive. Caterina tried to capture Cesare when he came near the fortress to talk to her, but the attempt failed. For several days the artillery of both factions engaged in a mutual bombardment: Caterina's cannon inflicted heavy losses on the French army, but the French artillery damaged the defences of the main fortress. What was destroyed during the day was rebuilt during the night. Caterina's solitary resistance was admired throughout all Italy; Niccolò Machiavelli reports that many songs and epigrams were composed in her honour. All were lost except that of Marsilio Compagnon. As time passed without decisive results, Cesare changed his tactics. His troops bombarded the walls of the fortress continuously, even at night. After six days, they opened two breaches in the walls.