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He was at first intended for a business career, but Lorenzo de' Medici, appreciating his ability, sent him as ambassador to various courts, where he acquitted himself with distinction. On the death of Lorenzo (1492), the succession of his weak and incapable son Piero led Capponi to become one of the leaders of the anti-Medicean faction which two years later succeeded in expelling Piero from Florence. Capponi was then made chief of the republic and conducted public affairs with great skill, notably in the difficult negotiations with Charles VIII of France, who had invaded Italy in 1494 and in whose camp the exiled Medici had taken refuge.
In November 1494 on his way to Naples, Charles entered Florence with his army, and immediately began to behave as though he were the conqueror of the city, because he had entered it lance in rest. The Republican Signoria was anxious to be on good terms with him, but when he spoke in favor of the Medici, their temper changed at once, and the citizens were ordered to arm and be prepared for all emergencies. Tumults broke out between French soldiers and Florentine citizens, barricades were erected and stones began to fly from the windows. This alarmed Charles, who lowered his tone and said nothing more about conquered cities or the Medici.
The Florentines were willing to pay him a large sum of money, but in settling the amount further disagreements arose. Charles, who was full the Medici's promises, made exorbitant demands, and finally presented an ultimatum to the Council of the Signoria, who rejected it. The king reputedly said that if it was declined, "Then we shall sound our trumpets". In response, Capponi is said to have ripped up the written demands before the King himself, and replied "And we shall toll our bells". Charles, who did not relish the idea of house-to-house fighting, was forced to moderate his claim and concluded a more equitable treaty with the republic.
On November 28 he departed, and Capponi was appointed to reform the government of Florence. But being more at home in the camp than in the council chamber, he was glad of the opportunity of leading the armies of the republic against the Pisan rebels. He proved a most capable general, but while besieging the castle of Soiana, he was killed on the 25 September 1496. His death was greatly regretted, for the Florentines recognized in him their ablest statesman and warrior.
- Saunterings in Florence: a new artistic and practical hand-book, by Elvira Grifi (1899), page 309.