After Pisa became an independent and free republic in 1494, the Florentines were unable to breach Pisa’s walls, nor were they able to block its trade, as the Arno River gave it access to the sea. Columbus had just returned from the Americas, and a few years later, Amerigo Vespucci would also return from Brazil. Amerigo Vespucci’s cousin, among other things, had worked with Machiavelli in the republic’s chancellery in Florence. The Americas made it more urgent for Florence to regain control of Pisa and access to the sea. In July 1503, Leonardo da Vinci was asked to divert the Arno River upstream to leave Pisa dry and give Florence access to the sea. Machiavelli also supported the idea. Leonardo planned everything in detail. One million tons of earth needed to be moved, requiring 1.3 million man-hours. Construction began in August 1504, but it proved inadequate, and a series of storms also destroyed the walls of the dams. The project failed and was abandoned in October 1504.
