During the Second Siege of Mainz Barrois was promoted to captain adjutant-major. Actually, Marceau's division and the 9th Light Infantry were engaged at the siege of Ehrenbreitstein Fortress from 15 September to 17 October 1795. Barrois fought at the Battle of Neuwied on 18 April 1797. He was transferred to Brest in order to take part in the 1798 expedition to Ireland. Later he was assigned to the Vendée. The 9th Light was assigned to the Army of Reserve and fought at the Battle of Marengo on 14 June 1800 as part of Jean Boudet's division. It performed so well that Napoleon had the word "incomparable" embroidered on the regimental flag. In October 1800, Barrois was appointed chef de bataillon.