With his return to the city of his youth, Leonardo did not abandon his engineering ambitions. He compiled a list of the scientific texts he would be able to consult in the monasteries of San Marco and Santo Spirito. Seeking distinction in the military field, he even sent a letter to Sultan Bajazeth, offering to design for him a bridge between Galata and Istanbul.

Thanks to Machiavelli, who had seen him at work with Cesare Borgia, Leonardo managed to establish himself as military engineer at Florence as well, called upon to serve the Republic during the war against Pisa, assigned the task of inspecting the fortresses that surrounded the enemy city. The project for deviating the course of the Arno, with which Leonardo suggested depriving the besieged city of the necessary supplies, was begun on that occasion, but due to the high costs involved the initiative was abandoned. Contemporaneously, Leonardo's return to painting was sanctioned by the commission assigned him by the Florentine government to paint the Battle of Anghiari, a gigantic wall painting representing the Florentines' victory over the Milanese in 1440, in the Sala del Maggior Consiglio at Palazzo Vecchio. The Florentine authorities were convinced that Leonardo could be much more useful to the cause of the Republic in the capacity of painter, producing a work imbued with propaganda, than as military technician. For the generations that came after him, the studies of the centrifugal motions of horses and the violent gestures and faces portrayed by Leonardo linked the artist's name to this painting, now lost, on which he worked fervently up to June 1505. And yet, having almost finished what would have been the most important work in the history of Italian art, Leonardo left Florence for Milan in 1506, at the request of the French authorities, abandoning work on the Battle.