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.
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.
Leonardo da Vinci took three months' leave of absence from his work on the Battle of Anghiari mural in Florence to go to Milan in the service of the French governor Charles d'Amboise, who commissioned the project of a house and garden from him.
A landslide occurred in the Alps near Bellinzona which is described by Leandro Alberti in 1550:
In the past years an earthquake caused a large part of a mountain to collapse, in such a way that the Bregno valley came to be obstructed; and as the river came to be dammed up, it produced a large and dark lake with great damage to the inhabitants of the valley, many of whom were drowned and their houses submerged. And so it stayed for quite some time, until the fallen earth made gradually soft by the infiltrations of the water and being no longer sufficiently strong to retain the immense pressure of it, suddenly burst open to the fury of the swollen waters. And since the former river bed, which was joined to the Ticino, was no longer adequate to contain it, the water flooded all the neighbouring regions, overthrowing in part even that strong wall which Lodovico Sforza had built near Bellinzona.
... for in our own times a similar thing has been seen, that is, a mountain falling seven miles across a valley and closing it up and turning it into a lake.