After a reorganization, Desjardin became lieutenant colonel of the 2nd Battalion of the "Maine-et-Loire" National Guard. He led his troops in the Army of the North at the Battle of Jemappes on 6 November 1792. This action was followed by the Siege of Namur, which lasted from early November until the Austrian surrender on 2 December. During the siege, Desjardin's battalion served in the Left Brigade of Louis-Auguste Juvénal des Ursins d'Harville's Reserve Division. In the actions during Charles François Dumouriez's subsequent retreat from Belgium, Desjardin greatly distinguished himself. On 3 September 1793, he received promotion to general of brigade. When he became a general of division on 19 March 1794, he already had charge of three divisions.