Charles William Ferdinand entered the military, serving during the Seven Years' War of 1756–63. He joined the allied north-German forces of the Hanoverian Army of Observation, whose task was to protect Hanover (in personal union with the Kingdom of Great Britain) and the surrounding states from invasion by the French. The force was initially commanded by the Anglo-Hanoverian Prince William, Duke of Cumberland. At the Battle of Hastenbeck (1757) Charles William Ferdinand led a charge at the head of an infantry brigade, an action which gained him some renown.