James FitzJames was the illegitimate son of James II (at the time, 1670, Duke of York) and his mistress Arabella Churchill, John Churchill's sister. James is better known to history as The Duke of Berwick, and ironically fought on different sides than his uncle, John Churchill, Duke of Marlborough, during The War of The Spanish Succession.
James joined his father and the Army in 1688 at Salisbury where James II was preparing to throw back his son in law's, William of Orange, invasion. As it happened too many deserted from the Royal Army and followed John Churchill in joining William's Army; Sarah Churchill, John's wife, being a prominent Orange supporter, and confidant of Princess (later Queen) Anne.
But Berwick did see action against William's forces not in England but in Ireland, and was present, and indeed wounded, at the Battle of The Boyne in 1690. He later took part in the sieges of Limerick and Cork, before joining his father in exile in France.
In France he eventually became a Marshal of France and arguably Louis XIV's greatest general. In 1707 he commanded a joint Franco Spanish Army at the battle of Almada in Spain. Here he faced and defeated a Frenchman, Henri de Massure, Marquis de Ruvigny and Earl of Galway. Henri was a French Huguenot (Protestant) who left France in 1691 for England where he served in William III's Army in Ireland, hence his Irish title of Earl of Galway. Berwick had the victory so that we can say a French Army led by an Englishman (even though Berwick was actually born in France) defeated an English Army (actually an Anglo Portuguese Army) led by a Frenchman.
Berwick's half brother, James the Old Pretender, wanted Berwick to lead the 1715 Rebellion. He refused, probably very wisely given the lack of planning, but James The Old Pretender never forgave him. When Charles Edward Stuart, The Young Pretender, launched the 1745 Rebellion, Berwick was eleven years dead, somewhat ruling him out as a commander in The '45. He had been decapitated in action by a cannon ball.
Today Berwick's descendants carry two Spanish titles, and formerly a French title too. The present senior member of the family is Carlos Fitz-James Stuart y Martinez de Irujo, 19th Duke of Alba. He does not carry the Jacobite title of Duke of Berwick because the succession laws of Spain and the Jacobites are different and in 1953 the title Duke of Berwick went to another member of the family, Jacobo Fitz-James Stuart, 12th Duke of Berwick. The Spanish House Stuart are descended from Berwick's first marriage. The French part of the family descended from Berwick's second marriage carried the title Duc de FitzJames. This family died out in 1967 and the title has therefore become extinct.