In 1289, the famous Battle of Campaldino took place between a Guelph coalition led by Florence and a Ghibelline force headed by Arezzo. Its notoriety is partly due to its mention in Dante’s Divine Comedy, where, in Canto V of Purgatory, the death of Bonconte da Montefeltro is recounted as a result of this very clash. It is believed that Dante himself fought in the battle as part of the Florentine cavalry.

During the summer of 1289, the Guelphs successfully launched a surprise offensive in the Casentino region, forcing the Ghibellines into open combat. Despite the Florentines’ numerical superiority, the tide initially turned in favor of the Aretines, whose cavalry charge disrupted the enemy's center. The Ghibelline infantry pushed forward and came close to breaking the Guelph lines. However, a surprising act of disobedience by a Guelph commander changed the course of the battle.

In medieval warfare, cavalry reserves played a vital role in pitched battles. Due to the lack of effective battlefield communication, commanders had few options to alter formations mid-fight, and deploying fresh troops at the right moment was one of the only ways to shift the outcome.

Corso Donati, leader of the hardline Guelph faction and future relative of Dante, who would marry into the Donati family, was in command of the Guelph reserves composed of troops from Pistoia and Lucca. A controversial figure, Donati is portrayed in historical accounts as a brave and skilled knight, yet also as cruel and deeply involved in political intrigue. He would later play a pivotal role in the internal strife between the Black and White Guelphs, facing multiple exiles throughout his life.

At Campaldino, Corso had been explicitly ordered not to engage his contingent under threat of execution. But seeing how the battle was unfolding, he chose to defy his orders and launched his troops in a charge against the right flank of the Aretine infantry. This maneuver prevented the Ghibellines from reinforcing the center. Deprived of infantry support, the Ghibelline cavalry lost momentum, was surrounded by the numerically superior enemy, and was ultimately annihilated.

In contrast, the commander of the Ghibelline reserves chose to abandon the battlefield without engaging, deeming the fight already lost. For Florence and the Guelphs, it was a resounding triumph. For Arezzo and the Ghibellines, it was a devastating defeat.