William inherited the principality of Orange from his father, William II, who died a week before William's birth. His mother, Mary, was the daughter of King Charles I of England. In 1677, he married his fifteen-year-old first cousin, Mary, the daughter of his maternal uncle James, Duke of York.
A Protestant, William participated in several wars against the powerful Catholic king of France, Louis XIV, in coalition with Protestant and Catholic powers in Europe. Many Protestants heralded him as a champion of their faith. In 1685, his Catholic father-in-law, James, Duke of York, became king of England, Ireland and Scotland. James's reign was unpopular with the Protestant majority in Britain. William, supported by a group of influential British political and religious leaders, invaded England in what became known as the "Glorious Revolution". On 5 November 1688, he landed at the southern English port of Brixham. James was deposed and William and Mary became joint sovereigns in his place. They reigned together until her death on 28 December 1694, after which William ruled as sole monarch.
William's reputation as a staunch Protestant enabled him to take power in Britain when many were fearful of a revival of Catholicism under James. William's victory at the Battle of the Boyne in 1690 is still commemorated by loyalists in Northern Ireland and Scotland. His reign in Britain marked the beginning of the transition from the personal rule of the Stuarts to the more Parliament-centred rule of the House of Hanover.
William III was born in The Hague in the Dutch Republic on 4 November 1650. Baptised William Henry (Dutch: Willem Hendrik), he was the only child of stadtholder William II, Prince of Orange, and Mary, Princess Royal. Mary was the eldest daughter of King Charles I of England, Scotland and Ireland and sister of King Charles II and King James II and VII.
Eight days before William was born, his father died of smallpox; thus William was the Sovereign Prince of Orange from the moment of his birth. Immediately, a conflict ensued between his mother the Princess Royal and William II's mother, Amalia of Solms-Braunfels, over the name to be given to the infant. Mary wanted to name him Charles after her brother, but her mother-in-law insisted on giving him the name William or Willem to bolster his prospects of becoming stadtholder. William II had appointed his wife as his son's guardian in his will; however, the document remained unsigned at William II's death and was void. On 13 August 1651, the Hoge Raad van Holland en Zeeland (Supreme Court) ruled that guardianship would be shared between his mother, his paternal grandmother and Frederick William, Elector of Brandenburg, whose wife, Louise Henriette, was William II's eldest sister.