In his masterpiece, The History of Italy, Guicciardini paved the way for a new style in historiography with his use of government sources to support arguments and the realistic analysis of the people and events of his time.

Influential in Florentine politics, Guicciardini's ancestors had held the highest posts of honour in the state for many generations, as may be seen in his own genealogical Ricordi autobiografici e di famiglia.

[6] Having distinguished himself in the practice of law, Guicciardini was entrusted by the Florentine Signoria with an embassy to the court of the King of Aragon, Ferdinand the Catholic, in 1512.

His Spanish correspondence with the Signoria[8][full citation needed] reveals his power of observation and analysis, a chief quality of his mind.

In his letters back home, he expressed appreciation for being able to observe Spanish military methods and estimate their strength during the time of war.

[4] As he later described himself during this period: "If you had seen messer Francesco in the Romagna...with his house full of tapestries, silver, servants thronged from the entire province where—since everything was completely referred to him—no one, from the Pope down, recognized anyone as his superior...".

As hostilities between King Francis I of France and Charles V, Holy Roman Emperor, escalated, the Pope remained undecided over which side to back and so sought Guicciardini's advice.

Guicciardini was powerless to influence the commander of papal forces, Francesco Maria I della Rovere, Duke of Urbino, to take action.

Guicciardini served three popes over a period of twenty years,[4] and perhaps because of his experiences, he was highly critical of the papacy: "I don't know anyone who dislikes the ambition, the avarice, and the sensuality of priests more than I do....

"[13] Like many Florentine aristocrats of his day, Guicciardini believed in a mixed republican government based on the model of the Venetian constitution;[14] despite working so often and closely with the Medici, he viewed their rule as tyrannical.

[15][full citation needed] Guicciardini was still able to reconcile his republican ideals and his support of the Medici: "The equality of men under a popular government is by no means contradicted if one citizen enjoys greater reputation than another, provided it proceed from the love and reverence of all, and can be withheld by the people at their pleasure.

"[16] Shortly after the Sack of Rome, Guicciardini returned to Florence, but by 1527, the Medici had been expelled from the city, and a republic had been re-established by the extreme anti-Medici Arrabiati faction.

Under the command of Clement VII, Guicciardini was assigned the task of punishing the Florentine citizens for their resistance to the Medici, and he dealt out justice mercilessly to those who had opposed the will of the Pope.

In that year, his descendants opened the Guicciardini family archives and committed to Giuseppe Canestrini the publication of his memoirs in ten volumes.

[23] These are some of his works recovered from the archives: Taken in combination with Machiavelli's treatises, the Opere inedite offer a comprehensive body of Italian political philosophy before Paolo Sarpi.

"[25] In the following excerpt, the historian records his observations on the character of Pope Clement VII: "And although he had a most capable intelligence and marvelous knowledge of world affairs, yet he lacked the corresponding resolution and execution.

For he was impeded not only by his timidity of spirit, which was by no means small, and by a strong reluctance to spend, but also by a certain innate irresolution and perplexity, so that he remained almost always in suspension and ambiguous when he was faced with those deciding those thing which from afar he had many times foreseen, considered, and almost revealed.

"[27] In the words of one of Guicciardini's severest critics, Francesco de Sanctis: "If we consider intellectual power [the Storia d'Italia] is the most important work that has issued from an Italian mind.