Duke Charles entered into the Prussian service at a young age. By the age of 19 he held a commission as a Captain and by 1806 he had been promoted to the rank Major. He fought for his brother in law King Frederick William III of Prussia during the War of the Fourth Coalition accompanying the king in his flight following the Prussian defeat in the Battle of Jena-Auerstedt. For his services in the Saale campaign Duke Charles was awarded the Order of the Black Eagle.