The Grand Duchy of Hesse and by Rhine (German: Großherzogtum Hessen und bei Rhein), or the Grand Duchy of Hesse (German: Großherzogtum Hessen) between 1806 and 1816, was an independent country and member state of the Confederation of the Rhine as of 1806, when the Landgraviate of Hesse-Darmstadt was elevated to a Grand Duchy which it remained until 1918, when the monarchy was overthrown. Hesse lost its independence when it joined the German Empire in 1871. Before 1866, its northern neighbour was its former sister Landgraviate, since 1803 an Electorate, of Hesse-Kassel – for this reason, this state was sometimes colloquially known as Hesse-Darmstadt.

Hesse-Darmstadt was a member of Napoleon's Confederation of the Rhine during the Napoleonic Wars. Rapidly expanding during the mediatizations, Hesse-Darmstadt became an amalgamation of smaller German states, such as the Electorate of Cologne. The legal patchwork of the state culminated in a decree issued on 1 October 1806 by Louis I. The old territorial estates were abolished, which altered Hesse-Darmstadt "from a mosaic of patrimonial fragments into a centralized, absolute monarchy." During the Congress of Vienna it was forced to cede the Duchy of Westphalia, which Hesse-Darmstadt had received in 1803, to the Kingdom of Prussia. However, Hesse-Darmstadt received some territory on the western bank of the Rhine, including the important federal fortress at Mainz.

...
Wikipedia