Medici, Cosimo I de', 1519–74, duke of Florence (1537–69), grand duke of Tuscany (1569–74); son of Giovanni de' Medici (Giovanni delle Bande Nere). In 1537, Lorenzino de' Medici murdered Cosimo's predecessor, Alessandro de' Medici, and fled from Florence, leaving the succession to Cosimo. Cosimo, despite promises to the contrary, assumed absolute authority as soon as he was installed. A group of exiles who tried to restore the republic were defeated and were either imprisoned or beheaded. In 1539, Cosimo married a Spanish noblewoman, Eleonora de Toledo, whose enormous dowry replenished his empty coffers. Under Cosimo's able, though ruthless, rule Florence reached its highest political importance and material prosperity and almost doubled its territories—notably by the acquisition (1555) of the republic of Siena. In 1569, Pope Pius V bestowed the title grand duke of Tuscany on Cosimo. Cosimo centralized his state. His son, Francesco de' Medici, succeeded him.
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