How did Cosimo I de’ Medici become duke of Florence?

Following the assassination of his distant cousin, Alessandro, the first duke of Florence, in January 1537, Cosimo I de’ Medici traveled immediately to the city. Upon his arrival, he was elected head of the republic, with the intention that he would be assisted in governance by the senate, the assembly, and the council.1

This election received the approval of the Holy Roman emperor Charles V. Shortly thereafter, Cosimo solidified his position by defeating a band of exiles at the Battle of Montemurlo in August 1537. Following this victory and with the continued support of Charles V, Cosimo began styling himself as duke in September 1537. Once he had secured control, the previously established governing bodies—the senate, assembly, and council—were rendered powerless, allowing Cosimo to establish his authority.1