The capital of this new state was Darmstadt, hence the name. As a result of the Napoleonic wars, the landgraviate was elevated to grand duchy following the dissolution of the Holy Roman Empire in 1806.
With the extinction of the Landgraviate of Hesse-Marburg and Landgraviate Hesse-Rheinfels lines by 1604, Hesse-Darmstadt, along with the Landgraviate of Hesse-Kassel (or Hesse-Cassel), became one of the two Hessian states.
While Hesse-Kassel converted to Calvinism and became one of the most zealous exponents of the Protestant cause in the Thirty Years' War, Landgrave George II of Hesse-Darmstadt remained a strict Lutheran and maintained a close alliance with Saxony, which resulted in a pro-Habsburg policy after 1635.
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.