In 1801, Camille Alphonse Trezel entered as a draftsman into the office of the war and in 1803 attained the rank of lieutenant in the body of topographical engineers. Posted in 1804 to Army of Holland, he was promoted the following year to assistant engineer geographer. After the Polish campaign, he was appointed acting aide to General Gardane at French embassy to Persia in 1807–1808, then aide to General Armand Charles Guilleminot on his return in 1809. He was secretary of the Committee on Delimitation of Illyria, was promoted to captain (1810), and transferred to the Spanish army. Recalled to Germany in late 1811, he worked at the Hanseatic surveying department, on the campaign to Rome, became lieutenant-commander (1813), Chief of Staff of the 13th Division, and worked on the defense of Mainz.