The Kiss by Gustave Klimt
In 1504, one of the most important artistic disputes in history took place in the Hall of the Five Hundred: Leonardo da Vinci and Michelangelo Buonarroti were summoned to fresco this important Palazzo Vecchio hall with scenes of two key battles in the history of the Florentine Republic—the Battle of Anghiari (la Battaglia di Anghiari) and the Battle of Cascina (la Battaglia di Cascina).
Leonardo was commissioned to paint The Battle of Anghiari and Michelangelo The Battle of Cascina. The artistic result of the affair was limited: Leonardo painted a beautiful fresco, but it was very delicate due to the experimental technique used, and Michelangelo never completed his work.
Dan Brown mentions this mysterious episode in Inferno:
Their bizarre history included a failed experimental painting technique by Leonardo da Vinci, which resulted in a “melting masterpiece.” There had also been an artistic “showdown” spearheaded by Piero Soderini and Machiavelli, which pitted against each other two titans of the Renaissance—Michelangelo and Leonardo—commanding them to create murals on opposite walls of the same room.
A curiosity about the Battle of Cascina, which was fought on July 29, 1364, between Florence and Pisa: Tradition says that the Florentines fought without armor. They had removed it because of the summer heat. Defeat was avoided thanks to a soldier who gave the alarm. This scene was very suitable for Michelangelo to show his skill in portraying male nudes. Michelangelo never finished the fresco because he was called to Rome by Pope Julius II.
Michelangelo created only preparatory drawings for the fresco: legend says that the drawings have been copied and greatly studied by other artists to the point of be spoiled. Actually, some preparatory drawings are preserved, but their attribution to Michelangelo is very uncertain. Giorgio Vasari and Benvenuto Cellini described these.
Cellini says:
Never among the ancients nor among modern there was a work that was so beautiful.
Instead, Leonardo created some part of the Battle of Anghiari, which was fought July 29, 1440, between Florence and Milano, but he used an experimental technique called encausto and the work could not be preserved. 
He placed two big pots full of burning wood in the lounge, creating a very high temperature that would have to dry the painted surface. The vastness of the work, however, did not allow the temperature to be sufficient to dry the colors, which flowed on the plaster, and almost disappeared.
After a few months, the artist stopped the work, frustrated by the failure. Despite this severe damage, The Battle of Anghiari was displayed for several years in the Palazzo Vecchio, allowing many artists to see and reproduce it. Rubens painting offers a pretty clear idea of what Leonardo’s fresco looked like.
A fascinating theory says that Leonardo’s work has been covered by new frescoes by Giorgio Vasari, who did not dare to delete it entirely. Thus, The Battle of Anghiari would still be hidden beneath Vasari’s paintings, and the enigmatic words “cerca trova” (seek and ye shall find) could be a clue.
Today, however, scholars and art historians do not believe this theory. You can read our post about “cerca trova” to learn more. The only evidence that remains of the two masterpieces by Leonardo and Michelangelo are the preparatory drawings and stories of the time, which mentioned the challenge between these two giants of the Italian Renaissance.
Here is a biography of Leonardo if you want to know more about:
Leonardo da Vinci: A Life by Sherwin Nuland
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Pictures from Wikipedia
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I just returned this week from visiting Florence and was in awe of the Uffizi and the Piazza della Signoria. Florence is an outdoor museum. Thank you for sharing this fascinating information. An author, Sylvain Reynard has opened my eyes again to the brilliance of Florence, Italy. His books, Gabriel’s Inferno, Gabriel’s Rapture and soon to be released Gabriel’s Redemption depicts the brilliance of these masters. I found Botticelli’s work breathtaking.
Thank you Ellie, that’s nice to hear so many compliments on our beloved city!