Orange-Nassau, also known as Nassau-Orange (German: Oranien-Nassau or Nassau-Oranien), was a principality which was part of the Lower Rhenish-Westphalian Circle within the Holy Roman Empire. It existed under this name between 1702 and 1815. The territory of the former state of Orange-Nassau is now part of Germany. It was ruled by the House of Orange-Nassau.
In 1702, the first House of Orange-Nassau became extinct with the death of William III, Prince of Orange, the Stadtholder in the Netherlands and King of England, Scotland and Ireland. John William Friso, Prince of Nassau-Dietz inherited the title "Prince of Orange" from his cousin, William III. From then on, the rulers used the title Fürst von Nassau-Oranien in Germany, and the title Prins van Oranje-Nassau (English: Prince of Orange-Nassau) in the Netherlands.
The principality soon became larger with the incorporation of other Nassau territories, due to the extinction of other branches of the House of Nassau. In 1711, the branch of Nassau-Hadamar died out. Although belonging to the remaining branches of Nassau-Siegen, Nassau-Dillenburg and Orange-Nassau, the principality of Nassau-Hadamar was not divided; it was provisionally administered by the ruler of Nassau-Dillenburg. When the branches of Nassau-Dillenburg and Nassau-Siegen died out in 1739 and 1743, all Nassau areas of the Ottonian Line were reunited and inherited by the branch of Orange-Nassau. The Prince of Orange-Nassau from then on had two seats in the Council of Princes of the Reichstag: Hadamar-Nassau and Nassau-Dillenburg.
By article 24 of the Treaty of the Confederation on 12 July 1806, William VI, Prince of Orange lost all the territories of the Principality of Orange-Nassau. The counties of Siegen, Dillenburg and Hadamar, and the Herrlichkeit of Beilstein, were incorporated into the Grand Duchy of Berg. By German Mediatisation, the county of Dietz and its dependencies, and the Lordships of Wehrheim and Burbach, all came under the sovereignty of the Duke of Nassau-Usingen and the Prince of Nassau-Weilburg. In 1808, the Prince of Orange also lost his rights as mediatized prince, and all his property was confiscated.
After the French troops were expelled from Germany in 1813, the Prince of Orange could retake the territories that were lost to the Grand Duchy of Berg in 1806. In addition, the following mediatised areas were added under his sovereignty: the Herrlichkeit of Westerburg, the Herrlichkeit of Schadeck, and that part of the county of Wied-Runkel that lay on the right bank of the river Lahn. On 26 November 1813, the Prince of Orange concluded a treaty with the Duchy of Nassau, in which the county of Nassau-Dietz was returned to the prince. The Amt Wehrheim, however, remained with the Duchy of Nassau.
However, the restoration was short-lived. On 31 May 1815, Prince William VI concluded a treaty at the Congress of Vienna with his Prussian brother-in-law and first cousin, King Frederick William III, by which he ceded the Principality of Orange-Nassau to the Kingdom of Prussia in exchange for Luxembourg, which was elevated to a Grand Duchy. On the same day, the Prussians gave most of the principality to the Duchy of Nassau (thereby uniting the areas of the Ottonian Line and the Walram Line of the House of Nassau). Only Siegen remained with Prussia.
In 1815, the prince became the new King of the Netherlands and Grand Duke of Luxembourg with the name of William I of the Netherlands. To this day, the Netherlands are ruled by descendants of the House of Orange-Nassau.