For approximately four years, from 1494 to 1498, a Dominican monk and preacher was first the conscience, then the virtual king of Florence. His admirers, indeed followers, included the Neoplatonic philosopher Pico della Mirandola, the young Machiavelli and the painter Botticelli. His two greatest enemies were just as eminent: Piero de' Medici, that feckless son of Lorenzo the Magnificent and heir to his father's dictatorship of the city, and that charming arch-sensualist Pope Alexander VI, among the most notorious of the notorious Borgias. The simple monk himself would pass into legend as the scourge of the rich and corrupt, a fanatical moralist, an accused heretic and, finally, a martyr.

Nowadays, though, people tend to recognize the name Girolamo Savonarola (1452-1498) chiefly because he instituted the original "bonfire of the vanities." In 1497, this crusading prior of San Marco (blessed with paintings by Fra Angelico) berated the Florentines over the fripperies they wore, the salacious books they read, the provocative paintings they hung on their walls, the gold and silver jewelry that flashed even in the half-light of the sacred cathedral, all the gaudiness, luxury and lasciviousness of their sinful lives. Away with these snares of the devil! And so, rather than celebrate the last day before Lent with a lewd carnival, Savonarola called for the faithful to cast their "vanities" onto a great pyramid of holy fire. And they did. According to the historian Jakob Burckhardt, a passing Venetian art dealer took a quick glance at the paintings on the as-yet-unlit bonfire and immediately offered 22,000 gold florins for the lot. To no avail.

Lauro Martines is one of our most renowned historians of the Italian Renaissance and of Florence in particular. His new book is, in some ways, a successor to April Blood, his account of the "Pazzi" conspiracy to assassinate Lorenzo de' Medici in 1478. Together the two volumes make up an engrossing study of society and politics during the Tuscan city's most illustrious half century. For Martines, Savonarola's success and ultimate destruction can best be understood in light of the internal dynamics of both the Florentine ruling caste and the city's unstable system of government.

It is a complex story, and the reader will need to slow down to follow its unfamiliar course. But the basic outline is this: Savonarola felt genuinely inspired by a prophetic conviction that Florence should become the center of a spiritual regeneration of the Christian world. Unquestionably, the Italy around him was rotten with hypocrisy, lechery and chicanery of every sort. Sacred offices were bought and sold, the Pope himself kept married women as mistresses and fathered bastards, the grandees of Florence set bravos on their enemies and then swaggered their way out of murder. Nonetheless, the people of the quattrocento were the spiritual children of the Middle Ages and, amidst the hurly-burly of the world's buying and selling, deep inside they still honored, even yearned for lives of goodness, virtue and purity.

Savonarola, born in Ferrara in 1452, had spent a number of years teaching logic and practicing his forensic skills before being sent to Florence in the 1490s. But once there his thrilling sermons galvanized the citizenry. Theirs was, after all, a city in crisis. When Piero de' Medici tried to buy off King Charles VIII's invading French army, he was essentially run out of town and the old-boy network established by his grandfather Cosimo and continued by his father Lorenzo was overturned. Florence set up a more democratic government, based on a Great Council that was composed not only of the rich but also of middle-class tradesmen. The Medici fiefdom became a republic. As Martines says, "The fall of the Medici had released the political ambitions of the popolo, and the popular thrust was now too strong to be turned back by the oligarchs, who did not dare openly declare that what they really wanted was an aristocratic state run by themselves."

Behind the scenes, then, various factions continued to scheme, some on behalf of the exiled Piero. Meanwhile Pope Alexander VI plotted and maneuvered to thwart the French attempt to annex the kingdom of Naples. But Florence refused to join his "Holy Alliance," largely because Savonarola viewed Charles as God's agent, sent to purify Italy like some Biblical warrior from the Book of Judges. Yet even as the friar was preaching austerity and the simple life, the city's merchants and bankers were losing customers and income, the upper classes were feeling increasingly hampered by this meddlesome priest, and the general population was suffering from famine and plague. Finally, in the wake of one too many public attacks on his papacy and character, Alexander ordered Savonarola to desist from preaching on pain of excommunication. He also threatened all Florence with interdiction.

Both man and city ignored the threat. Florence as a whole still clung to its prophet, the voice of righteousness crying in the wilderness of this world. However, Savonarola's enemies grew more shrewd. A Franciscan rival challenged him to compete in an ordeal by fire. He refused to take up the poisoned bait, but one of his disciples agreed to be, essentially, tried in a burning, fiery furnace. The Dominican and the Franciscan would both walk through 30 yards of raging fire, and God would certainly keep the Savonarolan unharmed. On the appointed day the piazza was packed with onlookers. The hour arrived, then passed, and no challenger appeared. Finally, the Dominican contingent left for home, no doubt with a secret sigh of relief. The restless people, however, had been denied their miracle, and Savonarola's enemies quickly pumped up their disappointment into resentment and anger. The next day the monastery of San Marco was attacked by an armed mob, combatants on both sides were slain, and Savonarola was taken into custody by a now largely alienated ruling class. Within short order, the once revered friar suffered torture (by the strappado, which results in the dislocation of the shoulders), duly "confessed" that he was a charlatan only interested in his personal advancement, and then publicly hanged, after which his body was burned and the ashes scattered into the Arno.

In recent years, Savonarola has sometimes been caricatured as a political and moral terrorist, but Martines refuses to accept this reductionist caricature. The friar certainly worked hard for the moral cleansing of a society that sorely needed it, and he seems to have been honorable, devout and sincere. As Martines reminds us, an old Medici watchword goes "omne nefas victis, victoribus omnia sancta" -- "All crimes to the losers, to the winners all things pure." In the end, while Savonarola may have burned "vanities," the city fathers of Florence, with the approbation of a dissolute and cynical pope, burned the man himself. There's fanaticism, and then there's fanaticism. ·

Michael Dirda is a critic for Book World. His e-mail address is mdirda@gmail.com, and his online discussion of books takes place each Wednesday at 2 p.m. on washingtonpost.com.