In Florence, Leonardo da Vinci and Niccolò Machiavelli become involved in a scheme to divert the Arno river, cutting the water supply to Pisa to force its surrender: Colombino, the project foreman, fails to follow da Vinci’s design, and the project is a major failure.
The mural for the Council Hall of Florence was commissioned from Leonardo with a contract of 4 May 1504, signed by Machiavelli as secretary of the Republic; but Leonardo had already begun working on the cartoon in the Sala del Papa in S. Maria Novella, which had been assigned to him on 24 October of the preceding year.
The mother of the next Duke of Urbino writes a letter of recommendation of Raphael to the Gonfaloniere of Florence: "The bearer of this will be found to be Raphael, painter of Urbino, who, being greatly gifted in his profession has determined to spend some time in Florence to study. And because his father was most worthy and I was very attached to him, and the son is a sensible and well-mannered young man, on both accounts, I bear him great love..."
Christopher Columbus returns to Spain from his fourth voyage, during which he and his younger son, Ferdinand, explored the coast of Central America from Belize to Panama.
Leonardo's work on the cartoon of the Battle of Anghiari was suddenly interrupted in the last months of 1504, and he went to Piombino on a somewhat official mission as a military architect. (During his absence Michelangelo was to receive the commission for the Battle of Cascina.) His mission was preceded by a diplomatic action conducted by Machiavelli himself. It can be inferred, therefore, that it was Machiavelli who suggested Leonardo's name for the programme of fortification projects suggested by Jacopo IV Appiano, Lord of Piombino, an ally of the Florentines at the time of the Pisian war, 1503-4, when Leonardo had already been consulted on the project of diverting the Arno River for strategic reasons, and when Antonio da Sangallo the Elder was the chief military architect of the Florentine Republic. Leonardo's activity at Piombino, revealed by newly discovered evidence, included the study of the city walls, the citadel and the main gate.