Appointed by Frederick I to the rank of field marshal in 1712, Leopold distinguished himself for his success during the War of the Spanish Succession. He was later appointed the commander of the Prussian-Saxon army during the Great Northern War against Sweden. Leopold was a personal friend of Frederick William I. The last great achievement of his military career was commanding the Prussian troops to victory over the Saxons at the Battle of Kesselsdorf in 1745 during the Second Silesian War.