of Orange (with his vague Lutheran childhood) stood up for the Protestant (mainly Calvinist) inhabitants of the Netherlands. Things went badly after the Eighty Years' War started in 1568, but luck turned to his advantage when Protestant rebels attacking from the North Sea captured Brielle, a coastal town in present-day South Holland in 1572. Many cities in Holland began to support William. During the 1570s he had to defend his core territories in Holland several times, but in the 1580s the inland cities in Holland were secure. William of Orange was considered a threat to Spanish rule in the area and was assassinated in 1584 by a hired killer sent by Philip. William was succeeded by his second son Maurits, a Protestant who proved an excellent military commander. His abilities as a commander and the lack of strong leadership in Spain after the death of Philip II (1598) gave Maurits excellent opportunities to conquer large parts of the present-day Dutch territory. In 1585 Maurits was elected stadtholder of the Provinces of Holland and Zealand as his father's successor and as a counterpose to Elizabeth's delegate, the Earl of Leicester. In 1587 he was appointed captain-general (military commander-in-chief) of the armies of the Dutch Republic. In the early years of the 17th century there arose quarrels between stadtholder and oligarchist regents—a group of powerful merchants led by Johan van Oldebarnevelt—because Maurits wanted more powers in the Republic. Maurits won this power struggle by arranging the judicial murder of Oldebarnevelt. 17th century Expansion of dynastic power Maurice died unmarried in 1625 and left no legitimate children. He was succeeded by his half-brother Frederick Henry (Dutch: Frederik Hendrik), youngest son of William I. Maurits urged his successor on his deathbed to marry as soon as possible. A few weeks after Maurits's death, he married Amalia van Solms-Braunfels. Frederick Henry and Amalia were the parents of a son and several daughters. These daughters were married to important noble houses such as the house of Hohenzollern, but also to the Frisian Nassaus, who were stadtholders in Friesland. His only son, William, married Mary, Princess Royal and Princess of Orange, the eldest daughter of Charles I of Scotland and England. These dynastic moves were the work of Amalia. Exile and resurgence Frederick Henry died in 1647 and his son succeeded him. As the Treaty of Munster was about to be signed, thereby ending the Eighty Years'