At the outbreak of the Franco-Prussian War, the Prince was given command of the Second Army, and defeated the French Army of the Rhine at the Battle of Mars-la-Tour on 16 August 1870, cutting off its escape route to the west. The battle was followed by another victory at Gravelotte-St.Privat on 18 August and the encirclement and annihilation of the Army of the Rhine at the Siege of Metz. After the fall of Metz on 27 October, his army was sent to the Loire to clear the area around Orléans, where French armies, first under Aurelle de Paladines, then under Chanzy, were trying to march north to relieve Paris. He won battles at Orléans on 2 December and Le Mans from 10–12 January 1871. For his services he was promoted to the rank of Generalfeldmarschall. After the war, the Prince was made Inspector-General and was given the rank of Field Marshal of Russia by Alexander II of Russia.