Landgraviate of Hesse-Kassel
John Ernst had an important military career. In 1684 he commanded a Dragoon regiment in the service of the Landgraviate of Hesse-Kassel. In 1688, at the start of the Nine Years' War, he defended Koblenz. Later he participated in the Siege of Mainz (1689), the Battle of Fleurus (1690) and the Battle of Landen (1693).