Ottavio Farnese, duke of Parma (1521 - 1586), the second son of Pierluigi Farnese, duke of Parma and Piacenza, married Margaret, the illegitimate daughter of Charles V, Holy Roman Emperor. Ottavio was 15 years old, Margaret, recently widowed by the death of Alessandro de' Medici, was 16. At first she disliked her youthful bridegroom, but when he returned wounded from the expedition to Algiers in 1541 her aversion was turned to affection.

Ottavio had become lord of Camerino in 1540, but he gave up that fief when his father became duke of Parma in 1545. After the Parmesan nobility assassinated Pierluigi Farnese in 1547, troops of the Emperor occupied Piacenza. Pope Paul III, father of the late Pierluigi and grandfather of Ottavio, attempted to regain Piacenza; he set aside Ottavio's claims to the succession of Parma, where he appointed a papal legate, giving him back Camerino in exchange, and then claimed Piacenza from the emperor—not for the Farnesi, but for the Church.

But Ottavio would not be put off: he attempted to seize Parma by force, and having failed, entered into negotiations with Ferrante Gonzaga. This rebellion on the part of his grandson hastened the pope's death, which occurred on 10 November 1349. During the interregnum that followed, Ottavio again tried to induce the governor of Parma to give up the city to him, but met with no better success; however, on the election of Giovan Maria Giocchi to the papacy as Julius III the duchy was conferred on him (1551).

This did not end Ottavio's quarrel with the Emperor Charles V, for Gonzaga refused to give up Piacenza and even threatened to occupy Parma, so that Ottavio was driven into the arms of France. Julius III, who was anxious to be on good terms with Charles V on account of the council of Trent which was then sitting, ordered Farnese to hand Parma over to the papal authorities once more, and on his refusal hurled censures and admonitions at his head, and deprived him of his Roman fiefs, while Charles did the same with regard to those in Lombardy. A French army came to protect Parma, war broke out, and Gonzaga at once laid siege to the city. But the duke came to an arrangement with his father-in-law, by which he regained Piacenza and his other fiefs The rest of his life was spent quietly at home, where the moderation and wisdom of his rule won for him the affection of his people. At his death in 1586 his son Alessandro Farnese, Duke of Parma and Piacenza succeeded him.

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