Hesse-Darmstadt gained a great deal of territory by the secularizations and mediatizations authorized by the Reichsdeputationshauptschluss of 1803. Most notable was the acquisition of the Duchy of Westphalia, formerly owned by the Archbishop of Cologne, as well as territories from the Archbishop of Mainz and the Bishop of Worms. In 1806, upon the dissolution of the Reich and the dispossession of his cousin, the Elector of Hesse-Kassel, the Landgrave took the title of Grand Duke of Hesse.
At the Congress of Vienna, the Grand Duke was forced to cede Westphalia to Prussia, in exchange for which he received a piece of territory on the Left Bank of the Rhine, including the important federal fortress at Mainz. The Grand Duchy changed its name to the Grand Duchy of Hesse and the Rhine in 1816
In 1867, the northern half of the Grand Duchy became a part of the North German Confederation, while the half of the Grand Duchy south of the Main remained outside. In 1871, it became a constituent state of the German Empire. The last Grand Duke, Ernst Ludwig (a grandson of Queen Victoria and brother to Empress Alexandra of Russia), was forced from his throne at the end of World War I, and the state was renamed the Free State of Hesse.
The majority of the state combined with Frankfurt am Main and the old Prussian province of Hesse-Nassau to form the new state of Hesse following the Second World War. The part of the state on the left bank of the Rhine forms part of the Rheinland-Pfalz state.
Landgraves of Hesse-Darmstadt
Grand Dukes of Hesse (and the Rhine) 1806-1918