[14] The Sienese called for help from King Manfred of Sicily, who provided a contingent of German mercenary heavy cavalry, as well as the Holy Roman states of Pisa and Cortona.

The first division, led by Count d'Arras, the seneschal of Count Giordano (the German mercenary commander), consisted of 200 German knights and 200 Sienese crossbowmen,[16] who maneuvered unseen south of the battlefield around the Monselvoli-Costaberci Ridge and took up a position south of Monselvoli Hill along the road to Asciano with orders to charge the Guelphs upon hearing the Ghibellines shout “San Giorgio" ("Saint George").

The result sowed caution in the Florentine commanders, who decided on a strategic withdrawal and had begun to break camp when the Sienese army crossed the Arbia River, probably on a bridge at Taverne d'Arbia early on 4 September, and began advancing.

[16] The Sienese-Ghibelline army waited until the sun was well up in order to deny that advantage to the enemy, and the battle finally started around 10:00 with a charge of the German knights, which forced the right wing of the Guelphs to withdraw.

The battle raged with the Florentines gradually gaining the upper hand due to the weight of superior numbers until about 15:00, when Niccolò da Bigozzi (in command of the Sienese 4th Division) attacked and stabilized the situation.

Around 18:00, the prearranged cry of “San Giorgio” went up from the Ghibelline ranks, and Count Arras led his German knights from ambush directly against the Florentine commander (who was probably Iacopino Rangoni da Modena, Uberto Ghibellino or Buonconte Monaldeschi – the chronicles differ)[16] and killed him.

As a result, Dante in his Divine Comedy reserved a place in the ninth circle of Hell for the traitor Bocca degli Abati.

[23] The Ghibelline commander Farinata degli Uberti is also consigned to Dante's Hell, not for his conduct in the battle, but for his alleged heretical adherence to the philosophy of Epicurus.