Hesse-Darmstadt gained a great deal of territory from the secularization and modifications authorized by the Reichsdeputationshauptschluss of 1803. The acquisition of the Duchy of Westphalia, formerly owned by the Archbishop of Cologne, was significant, as were the acquisition of 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.