He soon after displayed this energy and firmness of character in a very bad cause. The Florentines, unable any longer to endure the tyranny and vices of duke Alexander, appealed to Charles V., whom they regarded as lord paramount of their state. The emperor summoned Alexander to Naples, where he then was, to answer the charges made against him. He obeyed: but the emperor was so incensed that he began to fear the result, and was on the point of retreating, had not Guicciardini exhorted him to remain. He drew up a defence for him, and by a judicious distribution of bribes, succeeded in obtaining his acquittal; and Florence was again subjected to his yoke.
Two years after, Alexander was murdered by Lorenzino de' Medici, who considered that he had a better right to be considered the head of the family. But this act, undertaken without the participation of any accomplice, was not followed by the results that might have been anticipated. Lorenzino, frightened by his very success, fled the city, and his cousin Cosmo was raised to the supreme power, and afterwards named grand duke of Tuscany. Guicciardini assisted materially in his elevation, and hoped to be real chief of the state, while the other held the nominal rank. But Cosmo was of a crafty, cold, and ungrateful disposition, and treated his benefactor with such neglect, that he withdrew himself from public life, and retired to his country seat at Montici, in the neighbourhood of Florence.
From this time he occupied himself wholly in the composition of his history. It is a fine monument of his genius and industry. It commences with the invasion of Italy by Charles VIII., and goes down to the exaltation of Cosmo. The fault attributed to him as an author is prolixity, and to this he must plead guilty. He dwells with the most tiresome and earnest minuteness on the most trivial incidents; and the taking of an insignificant castle, followed by no important results, is attended by the same diffuseness and exactitude of detail as events of the greatest magnitude. But no historian surpasses Guicciardini when the subject is worthy of his pen. His animated descriptions of battles, the chances of war, and conduct of princes and leaders; his delineations of character, and masterly views of the course of events, all claim the highest admiration. The orations, which he intersperses, have been cavilled at, but they are eloquent, full of dignified exhortation, or sagacious reasoning. His account of the rise and formation of the temporal power of the popes excited great censure in catholic countries; and throughout he is accused of showing himself the enemy of the Roman church. It is true, that the pages of no other historian afford such convincing proofs of the pernicious effects resulting from the union of spiritual supremacy and temporal possessions. His powerful character of the infamous pope Borgia; his description of the fiery vehemence of Julius II.; his unveiling of the faults of Leo X., and the exposure he makes of the mistakes and weakness of Clement VII., present the very men and times to our eyes, and form as it were a school in which to study the philosophy of history. We perceive no partiality till the last few pages, which record the downfall of the republic of Florence. His language is, in the eyes of Italian critics, nearly pure; it is forcible, without being concise; and the clearness and majesty of the expressions in his best passages carry the reader along with him.