The Italian merchant prince Cosimo de' Medici (1389-1464) was the unofficial and benevolent despot of Florence, contributing much to making it the intellectual and cultural jewel of 15th-century Europe. The dynasty he founded ruled Florence until 1494.

Cosimo de' Medici was born on Sept. 27, 1389, the son of Giovanni de' Medici, who founded the family's legendary fortune, amassing enormous sums in trade and banking. After his father died in 1429, Cosimo continued the family's commercial and financial practices with great success. He brought goods of little weight and high value from the East and lent money to the princely houses of Europe.

Cosimo also adopted the policy, already traditional in his family, of supporting the lesser guilds and the poor against the wealthy aristocracy which ruled the city. These oligarchs became jealous of Cosimo's popularity and fearful of his democratic tendencies. Consequently they sought to destroy him and his family. In...