Inside da Vinci and Machiavelli’s Plot to Weaponize a River

In 1503, Leonardo da Vinci began painting a portrait of Lisa Gherardini del Giocondo, the wife of a wealthy Florentine silk merchant. With its enigmatic smile, the “Mona Lisa” would become one of the most famous paintings in history. But a closer look reveals something else. In the background is a winding waterway that some scholars believe may be the Arno, the Italian river that flows through Florence and Pisa on its way to the Mediterranean Sea. And while da Vinci was working on the portrait, he also was studying the river to devise an extraordinary plan to change its course.

Da Vinci was not the only Tuscan thinking about the Arno. Niccolò Machiavelli, second chancellor of the Florentine Republic who was later immortalized as the author of The Prince, was also exploring plans to radically alter the river. Their paths would converge in one of the most ambitious engineering schemes of the Renaissance.

At the dawn of the 16th century, Florence was a republic in crisis. To the west, Pisa had broken free of Florentine rule and controlled the lower Arno, cutting off Florence’s most direct route to the Mediterranean. Repeated attempts to retake the city had ended in failure. Meanwhile, a new threat emerged to the east. Cesare Borgia, the ambitious son of Pope Alexander VI, was carving out a state in the Romagna. 

Florence’s predicament was made all the more frustrating by the possibilities of a rapidly expanding world. News of Florentine explorer Amerigo Vespucci’s voyages to the New World had reached the city, fueling dreams of overseas commerce. Yet Florence’s access to the sea depended on Pisa.

Florence’s leaders searched for an alternative. If they couldn’t conquer Pisa outright, perhaps they could undermine it by transforming the landscape itself.

“The popular conception of Leonardo emphasizes his paintings, though he actually made very few in his lifetime,” says Leslie A. Geddes, associate professor of art history at Tulane University and author of Watermarks: Leonardo da Vinci and the Mastery of Nature. “He was primarily employed as what we might think of today as a military and civil engineering consultant.”

For nearly two decades, da Vinci served Ludovico Sforza, Duke of Milan, for whom he not only painted “The Last Supper,” but also devised plans for canals, flood-control measures and urban water systems. After Sforza’s regime collapsed, da Vinci began working for Borgia in 1502 as an architect and military engineer. 

Why da Vinci chose to work for Borgia, the Duke of Romagna, whom Machiavelli later held up as a model of political ruthlessness in The Prince, remains unclear. In Fortune Is a River, historian Roger Masters speculates that one possibility could have been that da Vinci went to serve as an informal Florentine informant. Whatever his motives, da Vinci spent about eight months working for Borgia, inspecting fortifications, surveying territory and producing maps, including a remarkably detailed map of the Arno River and its headwaters.

In October 1502, da Vinci and Machiavelli found themselves in the same place at the same time: Imola, a small town between Florence and the Adriatic Sea. Da Vinci was there as part of Borgia’s court; Machiavelli had been dispatched to monitor the duke’s ambitions and preserve Florence’s uneasy relationship with the leader.

Whether da Vinci and Machiavelli interacted directly in Imola remains unknown. But in a letter to Florence dated November 8, 1502, Machiavelli referred to an unnamed “friend” whose assessment of Borgia he considered especially worthy of attention. In his book, Masters raises the possibility that the mysterious figure might have been da Vinci. While no definitive proof has surfaced, Masters sees this as one possible explanation for how the two men so quickly found themselves working on the same ambitious military projects after returning to Florence.

One of their earliest shared projects was La Verruca, a fortress outside of Pisa. Machiavelli played a key role in its capture, while da Vinci, on June 21, 1503, was at the site as an adviser. Some scholars have even suggested that da Vinci incorporated a peak resembling Monte Verruca, home to the fortress, into the landscape behind the “Mona Lisa.”

Before long, both men were drawn into a far more ambitious undertaking. In July 1503, a report from the field recorded that “Leonardo da Vinci and some others” had examined a proposal concerning whether the “Arno turned there or remained with a channel” and deemed it “very appropriate.”

Two days later, the nature of the scheme became clearer. The account books of the Florentine government noted expenses for an expedition “with Leonardo” into Pisan territory “to divert Arno from its course and take it away from Pisa.”

Scholars generally distinguish between two related but separate Arno river projects. The first was a wartime plan to divert the lower Arno, depriving Pisa of water, trade and access to the river that sustained much of its economy. The second was far more ambitious. A long-term effort to make the Arno navigable by digging canals and carving tunnels through the hills west of Florence, potentially creating a direct route to the sea.

While historians broadly regard Machiavelli as a central figure in both projects, the extent to which he and da Vinci worked together on the Arno diversion remains contested. “We should not imagine Machiavelli and Leonardo as a pair of conspirators or close collaborators; there is no concrete evidence of such a deep personal connection,” says Emanuela Ferretti, an associate professor of architectural history at the University of Florence and a specialist in Renaissance architecture and engineering. “They were simply two extraordinary minds whose paths crossed in Florence during those years.”

“Machiavelli was one of the great political and strategic minds behind the project, viewing the diversion as the perfect weapon to break Pisa,” Ferretti explains. “Leonardo, on the other hand, provided his technical expertise by studying the course of the Arno and its water volumes, but his role was one of conceptual design.”

Whatever the exact nature of their relationship, da Vinci and Machiavelli were both associated with the scheme. The proposal called for diverting the Arno toward the marshlands near Livorno by means of weirs that would redirect the water into a mile-long ditch before splitting into smaller channels. Da Vinci estimated that roughly a million tons of earth would need to be moved, requiring some 54,000 man-days of labor.

“By the standards of the era, the undertaking was of staggering technical audacity. Diverting a river like the Arno meant defying the laws of nature using only the means available to the Renaissance: muscle power, shovels and geometric calculations,” Ferretti says.

However, the undertaking never achieved its goal. Although the proposal was approved in 1503, large-scale excavation didn’t begin until more than a year later. By then, responsibility had passed to other hands. “Leonardo’s direct involvement in the execution phase of the work is largely a historiographical myth—an idea that has solidified over time but lacks valid documentary evidence,” Ferretti says.

Under the direction of a water engineer known as Colombino, construction began in August 1504, differing in some respects from da Vinci’s original plans. The project consumed far more time and money than expected, and the diversion channels failed to carry the river as intended except during periods of flooding. As difficulties mounted, Florentine officials sought advice from the renowned architect Biagio Rossetti. According to Ferretti, Rossetti ultimately concluded that the enterprise couldn’t be salvaged, effectively bringing the project to an end.

With the diversion abandoned, so too was da Vinci’s broader vision of transforming the Arno into a navigable commercial artery linking Florence directly to the sea.

“Da Vinci’s pursuit of water’s complicated flow, movement and force—and how to marshal that knowledge for practical and artistic ends—occupied him throughout his career,” Geddes says. Even after the failure of the Arno project, da Vinci remained preoccupied with water, developing new schemes for canals and river navigation while filling his notebooks with studies of water’s movement and behavior.

Machiavelli, meanwhile, would draw on the political crises of the time to write The Prince, a work that helped make his name synonymous with political cunning.

As for the Arno itself, it still winds past Florence and out to the sea near Pisa, following much the same course it did five centuries ago.