At the Congress of Vienna, Hesse-Kassel was reestablished as the Electorate of Hesse. The neighboring Landgraviate of Hesse-Kassel had backed Prussia against Napoleon and was absorbed into the Kingdom of Westphalia. The Residenzschloss (city palace) of the Grand Dukes in Darmstadt However, Hesse-Darmstadt was compensated with some territory on the western bank of the Rhine, including the important federal fortress at Mainz. The old territorial estates were abolished, which altered Hesse-Darmstadt 'from a mosaic of patrimonial fragments into a centralized, absolute monarchy.' The Duchy of Westphalia, which Hesse-Darmstadt had received in 1803, was ceded to the Kingdom of Prussia during the Congress of Vienna. The legal patchwork of the state culminated in a decree issued on 1 October 1806 by Grand Duke Louis I. Rapidly expanding during the mediatizations, Hesse-Darmstadt became an amalgamation of smaller German states, such as the Electorate of Cologne. Hesse-Darmstadt was a member of Napoleon's Confederation of the Rhine during the Napoleonic Wars. It joined the German Empire in 1871 and became a republic following the empire's fall after World War I in 1918. Colloquially, the grand duchy continued to be known by its former name of Hesse-Darmstadt. After the end of the Napoleonic Wars in 1815, it changed its name in 1816 to distinguish itself from the Electorate of Hesse, which had formed from neighboring Hesse-Kassel. The grand duchy originally formed on the basis of the Landgraviate of Hesse-Darmstadt in 1806 as the Grand Duchy of Hesse (German: Großherzogtum Hessen). The Grand Duchy of Hesse and by Rhine (German: Großherzogtum Hessen und bei Rhein) was a grand duchy in western Germany that existed from 1806 to 1918.

Confederation of the rhine flag