Frederik Johan van Baer, Lord of Slangenburg (1645 - December 15 1713) was a Dutch military in service of William III of Orange.
He was a convinced Catholic, and therefore had no access to any administrative post.
A career in the Army was an obvious choise.
In 1665 he married Dorothea Petronella van Steenbergen, who died the same year.
Frederik Johan never remarried.
Van Baer rebuild his ancestral mansion the "Slangenburg" near Doetinchem into a considerable castle, fitting his status. His wife is remembered there in many paintings by Gerard Hoet.
Despite his faith, he had a very distinguished career in the Dutch Army, participating in the War of the Grand Alliance and the War of the Spanish Succession.
In 1675 he was the Colonel of one of the Dutch-Scottish regiments.
He became Major General in 1683, Lieutenant-General in 1692 and General in 1704.
In 1689 he commanded a wing at the Battle of Walcourt. In 1703 he saved a Dutch Army out of a desperate situation in the Battle of Ekeren.
Slangenburg resented the Duke of Marlborough and headed the opposition of certain Dutch generals against his command. In the end Slangenburg was removed in 1705 from his post and replaced by Ouwerkerk, who had a far better personal relationship with Marlborough.