He arrived in Florence in 1490 and became an instant hit when he preached. He railed against tyrants and their allies, the rich who exploited the poor. Using apocryphal language and backing his social and political views with I John and the Book of Revelation, his blend of populist preaching and sharp rhetoric earned him a name among the poor and enmity among the elite. Around 1493 he really began to turn up the heat and started to profess to have received visions and prophecies about a new Cyrus bringing the sword of the Lord. A year later it seemed as if the prophecy was coming true. King Charles VIII of France crossed the Alps and through the Italian peninsula into a tail spin.
Charles VIII marched onto Florence and briefly occupied the city after booting out the powerful Medici family who was ruling it at the time. Savonarola however had intense negotiations with the king that led France to continue their march south. A whole sack of cash promised by Savonarola might have helped Charles VIII in this decision. Savonarola preached a new Florence, one that was powerful and the center of a new empire of reforming the Church. He predicted spiritual and temporal riches, glory and might. So, now, a de facto an ally of Charles VIII it was no surprise when Pope Alexander VI sent Savonarola a letter demanding that Florence join the papacy in the war against the heretic Charles VIII that Savonarola decided not to oblige. The Pope excommunicated Savonarola.
With the threat of an interdict hanging over the heads of the Florentines, Savonarola was forced to back off the public stage. A rival suggested that Savonarola prove his prophecies, visions and spiritual mettle by walking through fire. When this didn't happen, public opinion turned against Savonarola. After this whole debacle Savonarola was arrested. While imprisoned he was tortured and he recanted his visions and prophecies. A few days later he was hanged.
The legacy of Savonarola however, is far reaching. His works were admired by Martin Luther, especially his thoughts on love and grace. Luther declared that he was a precursor and kindred thinker on Luther's revolutionary ideal of justification by faith alone. His works and sermons were translated into French and became a building block in the coming Huguenot reform. His political thinking and prophecy/vision thoughts were whitewashed a bit by the Dominicans and they revered him for his reformist zeal. Then much later, his puritanical bent was whitewashed and his political leanings were revived by the Il Risorgimento, leading to the Italian Unification in the 19th century. Savorna's teaching were used in at least two Italian political party formations in the 20th century albeit, tempered to fit their needs, but mostly focusing on his ideas of social justice. Today, the Catholic Church is considering him beatification. Not a bad little legacy you've got there.