Cosimo I de' Medici (12 June 1519 – 21 April 1574) was the second Duke of Florence from 1537 until 1569, when he became the first Grand Duke of Tuscany, a title he held until his death. Cosimo was born in Florence on 12 June 1519, the son of the famous condottiere Ludovico de' Medici (known as Giovanni dalle Bande Nere) and his wife Maria Salviati. He was the grandson of Caterina Sforza, the Countess of Forlì and Lady of Imola. Cosimo came to power in 1537 at age 17, just after the 26-year-old Duke of Florence, Alessandro de' Medici, was assassinated. Cosimo was from a different branch of the Medici family, descended from Giovanni de' Medici il Popolano, the great-grandson of Giovanni di Bicci de' Medici, founder of the Medici Bank. In 1537, Cosimo sent Bernardo Antonio de' Medici to Holy Roman Emperor Charles V to gain recognition for his position as head of the Florentine state. That recognition came in June 1537 in exchange for help against France in the course of the Italian Wars. With this move, Cosimo firmly restored the power of the Medici, who thereafter ruled Florence until the death of the last of the Medici ruler, Gian Gastone de' Medici, in 1737. The help granted to Charles V allowed him to free Tuscany from the Imperial garrisons and to increase as much as possible its independence from the overwhelming Spanish influence in Italy. Cosimo next turned his attention to Siena. With the support of Charles V, he defeated the Sienese at the Battle of Marciano in 1554 and laid siege to their city. Despite the inhabitants' desperate resistance, the city fell on 17 April 1555 after a 15-month siege; its population diminished from forty thousand to eight thousand. In 1559, Montalcino, the last redoubt of Sienese independence, was annexed to Cosimo's territories. In 1569, Pope Pius V elevated him to the rank of Grand Duke of Tuscany. In the last 10 years of his reign, struck by the death of two of his sons by malaria, Cosimo gave up active rule of the Florentine state to his son and successor Francesco I. He retreated to live in his villa, the Villa di Castello, outside Florence.

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