Alexander Farnese (Italian: Alessandro Farnese, Spanish: Alejandro Farnesio; 27 August 1545 – 3 December 1592) was an Italian noble and military leader, who was Duke of Parma, Piacenza and Castro from 1586 to 1592, as well as Governor of the Spanish Netherlands from 1578 to 1592. Nephew to King Philip II of Spain, he served in the Battle of Lepanto and the subsequent campaigns of the Holy League against the Ottoman Empire, being latter appointed general of the Spanish army during the Dutch revolt until his death in 1592. During the French Wars of Religion, he decisively relieved Paris for the Catholic League.

Farnese was regarded by contemporaries and historians as the greatest general of his age, as well as one of the best in history, for his talents as a commander, strategist and diplomat. His campaigns marked Spain greatest successes in the Eighty Years' War, giving the Spanish crown permanent control of the southern provinces, in the process establishing the cultural and religious separation which would eventually become the nation of Belgium.

Alessandro, born 27 August 1545, was the son of Duke Ottavio Farnese of Parma (a grandson of Pope Paul III) and Margaret of Austria (an illegitimate daughter of the Holy Roman Emperor and king of Spain Charles V). He had a twin brother, Carlo, who died in Rome on 7 October 1549. He and his mother, the half-sister of Philip II of Spain and Don Juan, left Rome for Parma in 1550. When Margaret was appointed Governor of the Netherlands, Alessandro accompanied her to Brussels in 1556 and was delivered to Philip II to ensure the loyalty of the Farnese. While in the King's custody, he visited the English royal court and then went to Spain to be raised and educated with his cousin, the ill-fated Don Carlos and his half-uncle, Don Juan, both of whom were about the same age as himself.

In 1565 his marriage with Maria of Portugal, which ended his stint as Philip II's hostage, was celebrated in Brussels with great splendour. He commanded three galleys during the Battle of Lepanto (1571) and the subsequent campaigns against the Turks. It was seven years before he again had the opportunity to display his great military talents. During that time the provinces of the Netherlands had revolted against Spanish rule. Don Juan, who had been sent as governor-general to restore order, found difficulties in dealing with William the Silent, who had succeeded in uniting all the provinces in common resistance to King Philip II.

In the autumn of 1577 shortly after his wife's death, Farnese led Spanish reinforcements from Italy along the Spanish Road to join Don Juan, and it was his able strategy and prompt decision at a critical moment that decisively won the Battle of Gembloux in early 1578. Shortly thereafter was the Siege of Zichem (Dutch) where the garrison was put to the sword and the town was sacked. This incident is considered to be the biggest stain on Farnese's otherwise chivalrous career. Next was the five-day Siege of Nivelles (Dutch) (French) in March 1578. The example of Zichem encouraged the citizens to capitulate so quickly. That summer, Farnese managed to prevent the defeat at the Battle of Rijmenam from being decisive.

In October 1578, Don Juan, whose health had broken down, died. Phillip II appointed Farnese to take his place as Captain-General of the Army of Flanders and appointed his mother Margaret as Governor-General. This was unacceptable to Alexander; he demanded to be both Captain-General and Governor-General or he would resign thus leaving military matters entirely in Margaret's hands. Philip eventually capitulated and after four years, Margaret returned to Parma.

Upon Don Juan's death, Farnese was confronted with a difficult situation. Perceiving that his opponents were divided between Catholic and Protestant, Fleming and Walloon, he skillfully worked to exploit these divisions. By this means, he regained the allegiance of the Walloon provinces for the king. Through the treaty of Arras, January 1579, he secured the support of the "Malcontents" (the Catholic nobles of the south) for the royal cause. The rebels in the seven northern provinces then formed the Union of Utrecht, formally abjuring Phillip's rule and pledging to fight to the end.

As soon as he had secured a base of operations in Hainaut and Artois, Farnese set himself in earnest to the task of reconquering Brabant and Flanders by force of arms, beginning with Maastricht. Farnese commenced the Siege of Maastricht on 12 March 1579. He ordered his troops to sap the walls. The inhabitants of Maastricht were also digging to reach the Spanish tunnels. Deep underground the fighting continued, hundreds of Spanish soldiers died as boiling oil was poured into their tunnels. Others died because of a lack of oxygen when the Dutch defenders ignited fires within them. Another 500 Spanish soldiers died when a mine, which they planned to use to blow up the wall, exploded prematurely.