Nassau (state) - Duchy of Nassau

On July 17, 1806 the remaining counties of Nassau-Usingen and Nassau-Weilburg joined the Confederation of the Rhine. Under pressure from Napoleon both counties merged to the Duchy of Nassau on August 30, 1806 under joint rule of Frederick August, Prince of Nassau-Usingen, and his younger cousin Frederick William, Prince of Nassau-Weilburg. As Frederick August had no heirs he agreed that Frederick William should become sole ruler after his death. However Frederick William died from a fall on the stairs at Weilburg Castle on 9 January 1816 and it was his son William who became Duke of Nassau.

At the 1815 Congress of Vienna the Principality of Orange-Nassau was incorporated into the Duchy of Nassau and subsequently the unified duchy joined the German Confederation with Wiesbaden as its new capital. Through the extinction of most lines, the Nassau-Usingen branch of the Nassau-Weilburg line under Duke William became the reigning house of Nassau until after the Austro-Prussian War in 1866 it was annexed by the Kingdom of Prussia and incorporated into the Province of Hesse-Nassau. William's son Adolphe, last Duke of Nassau, received Luxembourg after the male line of Nassau-Dillenburg became extinct in 1890.