After returning to France, Cassagne was assigned to defend Maubeuge until 1804 when he was sent to Bruges. He became an officer of the Legion of Honor on 4 June 1804. His regiment was placed in Charles-Étienne Gudin de La Sablonnière's division in the III Corps. He fought in the War of the Third Coalition in the Austrian Empire in 1805 and in the Kingdom of Prussia and Poland in 1806–1807. At the Battle of Auerstadt, the 25th Line Infantry Regiment captured two cannons from the Prussians and Cassagne was hit in the face by a bullet and had a horse killed under him. The 25th Line fought at the Battle of Pułtusk on 26 December 1806 and at the Battle of Eylau on 8 February 1807. Cassagne was promoted general of brigade on 7 June 1807. Napoleon raised him to the dignity of Baron of the Empire on 8 March 1808.