|The battle of Montaperti was an episode in the perpetual
struggle between Tuscan Guelf and Ghibelline factions that had kept the
region in turmoil since 1215. The Florentine Guelfs had established their
temporary hegemony in the province in the early 1250s, but their Ghibelline
enemies were encouraged by the military victories in the south of Frederick
II's illegitimate son, Manfred, who was crowned King of Sicily in Palermo
A group of Florentine Ghibellines, under the leadership of Farinata degli Uberti, fled to Siena and encouraged their allies in that city to resist Florence, and to enlist support from Manfred. A contingent of German cavalry arrived in Siena in the summer of 1259, bolstering the local Ghibelline forces and raising their hopes for victory over their Guelf rivals, In May, 1260, a Florentine army attempted a surprise attack on Siena, but it was repulsed. A second force, reinforced by Guelf contingents from other Tuscan towns - Lucca, Pistoia, Prato, Arezzo, Volterra, San Gimignano - was assembled in August. The size of that Guelf army has been estimated at 70,000, of which 16,000 were Florentines; not since Roman times had a military force of that magnitude been seen in central Italy.
The Florentine host established its camp near the castle of Montaperti, 12 kilometers (seven miles) east of Siena. And there, on the morning of 4 September, the Sienese and their Florentine and German allies, numbering perhaps 25,000, attacked and vanquished the vastly superior Guelf army. Some 10,000 died in the battle, and 20,000 more were taken prisoner and led back to Siena, where many died in Prison.
from Brucker, Florence: The Golden Age
This battle was immortalized by Dante:
"Now tell me (may you regain the sweet world's vantage),
Why is that people so fierce in its decrees
Toward my kin?" I answered, "It was the carnage
And devastation that dyed the Arbia red
Which made the prayers in our temple savage."
Dante, Inferno, 77-86, Pinsky translation