Even while he was working on his poetry, Dante kept an eye on Florentine affairs. In 1289, Dante enlisted to fight on the side of the pro-Pope Guelphs against the pro-Emperor Ghibellinis, a battle that the Guelphs won. Pleased with his performance on the battlefield, Dante decided to become a politician. In 1295, he joined the apothecaries' guild, which accepted writers and philosophers. Once he completed this entry requirement into Florentine politics, he rose swiftly through the ranks. He was elected to the council that selected the Priors of Florence, a panel of six magistrates who were the highest governing body in the land. By 1300, Dante had become a Prior himself.
Florence was then engulfed in a dramatic feud between two of its most prominent families, the Donatis and the Cerchis. It was a violent beef in which it was almost impossible for a Florentine to stay neutral. One young man even got his nose cut off during one of their fights. As tensions between the two sides escalated, the Guelphs split into two camps, the pro-Donati Blacks and the pro-Cerchi Whites.
Dante was a White Guelph, mostly out of loyalty to his friend Guido Cavalcanti, a strong supporter of the Cerchis. (Dante's wife, however, was of the Donati family. The whole thing is really complicated. Medieval Florence was like the final season of The OC.) He chose sides reluctantly. Dante felt that Florence's endless political squabbles were stupid. Government, he believed, should devote its efforts to building infrastructure - things like roads and public works - rather than arguing over who-cut-whose-nose-off. Unfortunately, the rest of the Florence political establishment didn't share his opinion. And really unfortunately for Dante, Pope Boniface VIII was among the biggest schemer of them all.
In the Middle Ages, the Pope held a position more similar to a king or an emperor than a religious leader. The Pope was tremendously powerful, wielding political as well as military influence. Some Popes, like Boniface, chose to exploit that power more than others. In 1301, shortly after Dante was made a Prior, Boniface decided that he wanted control of some lands south of Siena. A cardinal sent a request on Boniface's behalf to the Priors of Florence, asking for 200 mounted troops to go fight for control of the land the Pope wanted. The rest of the Priors either supported the Pope's request or maintained their silence. Dante alone spoke out against it. When word of Dante's resistance got back to Boniface, the Pope was infuriated. From then on, Dante's political career was in serious trouble.
The Pope's next shady move was to send a French envoy known as Charles de Valois to Florence in order to secure Boniface's power there. Charles de Valois was a seriously nasty man with a reputation for violence. In October 1301, Dante left Florence as part of a three-man delegation to Rome to beg the Pope to call off this awful man. Little did Dante know that he would never see the city of his birth again.
The delegation was too late. By 1 November 1301, Charles de Valois had already entered Florence with his men, setting off a campaign of looting and burning. Florentine politics were shaken to the core. All of the sitting Priors (including Dante, who was still on the road) were ousted. A new, all-Black set of Priors was ushered in. On 27 January 1302, Dante learned that the new priors had charged him with barratry, the act of accepting money for political favors. They banned him from entering Florence for two years and from public office for life. He was also ordered to pay a fine. Dante ignored the charges. On 10 March 1302, Dante and fourteen other men were sentenced to death. Thus began Dante's exile, which lasted for the rest of his life. In Paradiso, an ancestor of Dante's warns him how his life is going to turn out: "You shall abandon everything you love most dearly/ That is the arrow which the bow of exile shall shoot."