Having been promoted to first lieutenant Decouz joined the staff of François Rambeaud in 1797. He participated in the French campaign in Egypt and Syria where he fought at the Battle of the Pyramids in 1798 and was promoted captain on the battlefield. He was promoted again to "chef de bataillon" during the Siege of Acre in 1799. He became aide-de-camp to Jean Lannes and distinguished himself at the Battle of Abukir. Lannes sent him on an important mission to the Pasha of Syria after which Decouz became an adjutant lieutenant colonel. He then transferred to Louis Friant's staff as aide-de-camp.