After the Austrian service and the Peace of Lunéville, Wardenburg garrisoned in Bohemia until 1805. Then his patron, Duke Peter Friedrich Ludwig of Oldenburg, brokered a transfer to the Russian army, where Wardenburg joined the Azov Regiment as a lieutenant. He fought in Austria in 1805, in East Prussia in 1807, and against Sweden in Finland in 1808, taking wounds at each stop. In 1810 he became adjutant to Prince Peter Friedrich George of Oldenburg, who was serving as a Russian governor, and after Napoleon invaded Russia in 1812 Wardenburg was attached to the staff of the Russian commander-in-chief. In 1813 he joined the Russian-German Legion, a unit Duke Peter Friedrich Ludwig had personally organised. He was a lieutenant colonel in May 1814 and a colonel by January.

Oldenburg had no real army to speak of when Wardenburg arrived back in August 1814 as a freshly minted Oldenburg colonel. There was the Guard Company he had left in 1799, and not a great deal else. He raised the Oldenburg Regiment in time to march it against Napoleon during the Hundred Days campaign of 1815, then turned to the harder, slower work that occupied the rest of his career. He started a military school, founded the Grand Ducal Oldenburg Military Library to train officers and non-commissioned officers, and oversaw the construction of the infantry barracks at the Pferdemarkt that today houses parts of the city administration. The Duke kept the budget tight. Wardenburg's plans were repeatedly cut back by Peter Friedrich Ludwig's strict austerity.

When Grand Duke Paul Friedrich August acceded in 1829, he turned out to be genuinely interested in the army, and on the last day of that year Wardenburg was promoted to major general. The Federal War Constitution of 1820-21 required the small German states to pool troop contingents, and in 1834 Oldenburg signed a military convention with the Hanseatic cities of Hamburg, Lübeck, and Bremen. The Oldenburg-Hanseatic Brigade that resulted brought together infantry from all four contracting parties. Wardenburg's Oldenburg Half-Brigade, two infantry regiments plus an artillery unit, were the contribution he had spent twenty years building. He became the combined brigade's first commander and led it until his death on May 29, 1838, eleven days after his fifty-seventh birthday.

Wardenburg's other career was strange for an active-duty officer. He worked as a historian and a collector of antiquities, particularly artefacts dug up from old fortifications, and his collection became the founding nucleus of the Grand Ducal Collection of Antiquities and the Museum of Natural History and Prehistory in Oldenburg. Both of those institutions evolved into what is now the State Museum for Nature and Man, still operating in the city today. He also wrote military history, including a published study of his own regiment's part in the 1815 Summer Campaign against Napoleon. His marriage in 1816 to Helene Elisabeth Wilhelmine Hegeler remained childless, so the books, the papers, and the artefact collection were what survived him.

Wardenburg's personal estate is held by the Lower Saxony State Archives, Oldenburg branch, and his documentation of the Oldenburg Regiment in France went to the Oldenburg State Library. The infantry barracks he built at the Pferdemarkt still stand and still serve municipal purposes, though most of the soldiers who used to march in their courtyards belonged to regiments that no longer exist. A street in Oldenburg is named for him, Wardenburgstrasse, a short stretch in the city's grid. His memorial stone stands in the churchyard of St Gertrude in Oldenburg, a quiet plot to one side. The museum he founded by accident, now the State Museum for Nature and Man, still displays objects from the collection he assembled while building an army for a state that has not existed for nearly eighty years.