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In Milan and Naples (1555–59)

In later years, the focus of conflict between France and Spain had moved to the Italian peninsula. Alba was sent to Italy as commander in chief of the Spanish-Habsburg army in Italy, and became governor of Milan in 1555, and viceroy of Naples in 1556.

The newly appointed Pope Paul IV, an enemy of the Habsburgs, prompted King Henry II of France to expel the Spanish from Italy. Papal troops joined the French for this aim. In July 1556 the Pope declared Philip II was removed from the title King of Naples. Alba did not hesitate and marched on Rome at the head of 12,000 Spanish soldiers. He financed the campaign, in part, by obtaining a loan of 430,000 ducats from Bona Sforza, dowager Queen of Poland; the loan became known as Neapolitan sums and was never repaid. The Pope called for a truce, giving time for a French army commanded by Francis, Duke of Guise to march on Naples. The Spanish intercepted the French and defeated them in the Battle of San Quentin. Without French support, the papal troops were overwhelmed by the Spanish and the Duke of Alba entered Rome in September 1557. The pope had to sue for peace.

In April 1559, Alba was one of the signatories of the Treaty of Cateau-Cambrésis which ended their war with France and released Spanish resources for maximising its economic exploitation of New Spain. The Italian peninsula entered a prolonged period of peace, sealed by marriage between the twice widowed Philip II and Isabel de Valois, daughter of Henry II of France. During the royal wedding, which was held in Paris, Alba acted as proxy for Philip.