Decouz led his regiment at the Battle of Pułtusk on 26 December 1806. The 21st Line also fought at the Battle of Eylau on 8 February 1807. He became a Baron of the Empire on 27 November 1807. In 1809, his regiment fought at the battles of Eckmühl, Ratisbon and Aspern-Essling. During the operations leading up to the Battle of Wagram, Decouz led the seizure of an island in the Danube River in which his men captured 600 Austrians, Colonel Saint-Julien and many artillery pieces. This feat earned him promotion to general of brigade on 12 July 1809 and the commander's cross of the Legion of Honor on 21 September 1809.