Ernest was brought up as a Calvinist during the Thirty Years' War. He made his Grand Tour to France and Italy, and fought with Hesse-Kassel during the final years of the war, for example at the Battle of Nördlingen on 3 August 1645. In 1647, the army of Landgravine Amalie Elisabeth reconquered Lower Katzenelnbogen and returned it to Hesse-Kassel. In 1649, Ernest came of age and received Lower Katzenelnbogen. This made him the founder of the Hesse-Rheinfels line. Hesse-Rheinfels was not considered sovereign: it remained under the sovereignty of Hesse-Kassel, as did the other parts of the Rotenburg Quarter. Details of the relationship between Hesse-Rheinfels and Hesse-Kassel were laid down in a series of house treaties; nevertheless, political and judicial disputes often arose between the two houses.