William I, Elector of Hesse was the eldest surviving son of Frederick II, Landgrave of Hesse-Kassel and Princess Mary of Great Britain, the daughter of George II.

Frederick II was Landgrave of Hesse-Kassel from 1760 to 1785. He ruled as an enlightened despot, and raised money by renting soldiers to Great Britain to help fight the American Revolutionary War. He combined Enlightenment ideas with Christian values, cameralist plans for central control of the economy, and a militaristic approach toward international diplomacy.