All the republican faction, who saw in any sort of revolution the means of subjugating Florence, joined their party, set the captives at liberty, and seized Guglielmo; then proclaiming the establishment of the ancient constitution, they besieged the citadel, whither Cosimo dei Pazzi, Bishop of Arezzo, the son of Guglielmo, had fled for refuge; he, finding himself invested on every side, sent a messenger in hot haste to Florence to ask for help.
Unfortunately for the cardinal, Vitellozzo's troops were nearer to the besiegers than were the soldiers of the most serene republic to the besieged, and instead of help--the whole army of the enemy came down upon him.

This army was under the command of Vitellozzo, of Gian Paolo Baglioni, and of Fabio Orsino, and with them were the two Medici, ever ready to go wherever there was a league against Florence, and ever ready at the command of Borgia, on any conditions whatever, to re-enter the town whence they had been banished.

The next day more help in the form of money and artillery arrived, sent by Pandolfo Petrucci, and on the 18th of June the citadel of Arezzo, which had received no news from Florence, was obliged to surrender.
Vitellozzo left the men of Arezzo to look after their town themselves, leaving also Fabio Orsina to garrison the citadel with a thousand men.
Then, profiting by the terror that had been spread throughout all this part of Italy by the successive captures of the duchy of Urbino, of Camerino, and of Arezzo, he marched upon Monte San Severino, Castiglione, Aretino, Cortone, and the other towns of the valley of Chiana, which submitted one after the other almost without a struggle.
When he was only ten or twelve leagues from Florence, and dared not an his own account attempt anything against her, he made known the state of affairs to the Duke of Valentinois.

He, fancying the hour had came at last far striking the blow so long delayed, started off at once to deliver his answer in person to his faithful lieutenants.
But the Florentines, though they had sent no help to Guglielmo dei Pazzi, had demanded aid from Chaumont dumbest, governor of the Milanese, an behalf of Louis XII, not only explaining the danger they themselves were in but also Caesar's ambitious projects, namely that after first overcoming the small principalities and then the states of the second order, he had now, it seemed, reached such a height of pride that he would attack the King of France himself.

The news from Naples was disquieting; serious differences had already occurred between the Count of Armagnac and Gonzalva di Cordova, and Louis might any day need Florence, whom he had always found loyal and faithful.