A staunch ally and supporter of Emperor Napoleon I of France, Poniatowski voluntarily took part in the French invasion of Russia of 1812. Injuries received during the fighting for Moscow eventually forced his return to Warsaw, where he worked on the reconstruction of the Polish forces intended to fight in Germany. Covering the retreat of the French army after Napoleon lost the "Battle of the Nations" at Leipzig (1813), Poniatowski was repeatedly wounded and drowned in the Elster river.