The House of Nassau-Weilburg, a branch of the House of Nassau, rules a division of the County of Nassau, which is a state in what is now Germany, at this time part of the Holy Roman Empire, from 1344 to 1806.
On July 17, 1806, on the dissolution of the Holy Roman Empire, the counties of Nassau-Usingen and Nassau-Weilburg both join the Confederation of the Rhine.
Under pressure from Napoleon both counties merge to become the Duchy of Nassau on August 30, 1806, under joint rule of Prince Frederick August of Nassau-Usingen and his younger cousin Prince Frederick William of Nassau-Weilburg.
