Dante Alighieri was born into a Guelph family, and at the age of about 24 he fought in the Battle of Campaldino between Florence and her allies against Arezzo. The catalyst for the war is unknown: an account many years later from a Florentine claimed there were "outrages" committed by Arezzo. Retaliation by the Guelphs over these "outrages" caused Arezzo to gather a military force to oppose them.
There was a rumor that the bishop of the see of Arezzo was going to turn the commune over which he had authority (a place called Bibbiena Civitella) and connected villages to Florence for the price of 5000 gold florins annually. Arezzo forced this bishop onto a horse and led him to the battlefield from which, not surprisingly, he did not return.
The wealth of Florence enabled them to have a force that was superior in numbers (about 12,000, of which 10,000 were infantry), armor, and weaponry. The Ghibelline force was smaller but better trained, consisting of feudal lords and their military retinues, rather than paid volunteers as in Florence and her allies.
The Florentines were also hampered by their leader. The various communities from which the troops were drawn could not decide who should lead them, so they agreed on a mercenary, Aimeric IV, Viscount of Narbonne. He had distinguished himself as a fighter, but did not have much experience as a leader. He came to Italy in the service of Charles I of Anjou, but suffered from a serious impediment: he did not speak Tuscan Italian, making the relaying of his orders delayed as they had to be translated, as did the news for him from others of what was happening in different parts of the field.
Ultimately, the Guelphs won with superior numbers, and Florence was able to exert much more influence over more of the Italian peninsula on behalf of the papacy. Without a strong mutual enemy in Ghibellines anymore, however, the Guelphs fractured. The Black Guelphs continued their support for the power and authority of the popes, whereas the White Guelphs wanted more freedom from Rome.
The Battle of Campaldino was fought in June of 1289. Dante, a member of the White Guelphs, went on to hold some political offices, including prior of Florence. In 1301 he was part of a delegation to Rome to determine the intentions of Pope Boniface VIII toward the French ambassador Charles of Valois, the brother of King Philip IV. (Philip and Boniface had clashed over the topic of taxation.)
When the pope dismissed the rest of the delegation, he told Dante he had to stay. This did not turn out well, as I'll explain tomorrow.