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How did Florence become part of the Grand Duchy of Tuscany?

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Florence The Florence Cathedral, also known as the Duomo or the Cathedral of Santa Maria del Fiore, in Florence.

Florence became the center of the Grand Duchy of Tuscany through the consolidation of power by Cosimo I de' Medici during the 16th century.12

Following the assassination of Duke Alessandro de' Medici in 1537, Cosimo I was elected head of the Florentine government with the approval of the Holy Roman emperor Charles V. Cosimo I subsequently pursued a policy of territorial expansion, most notably launching a successful military campaign against the republic of Siena. By 1557, he had annexed Siena, bringing nearly all of Tuscany under his control.23

In 1569, Pope Pius V officially conferred the title of grand duke of Tuscany upon Cosimo I, formalizing the transition from a city-state ruled by a duke to a grand duchy. This established a new, authoritarian dynasty that governed Florence and the surrounding region until the Medici family line ended in 1737.2314