William was born in The Netherlands as Prince William Henry of Orange. His mother was Mary Stuart. Mary was the sister of the King of England, James II, so King James was William's uncle. The people of England did not like King James, and he was overthrown in the "Glorious Revolution" of 1688. King James escaped to France. William became the new King of England and married James' daughter, making her Mary II of England.
William's enemy was the King of France, Louis XIV, who protected and supported King James in his efforts to return.