At war's outbreak Debray became an aide-de-camp to Texas governor Edward Clark as a major in the 2nd Texas Infantry Regiment. Debray raised a cavalry unit from Bexar County, was elected commander of Debray's Texas Cavalry Battalion, and was commissioned colonel on December 5, 1861. He led the unit which became the 26th Texas Cavalry at the Battle of Galveston Bay in 1862 and later during the Red River Campaign serving with distinction at the Battles of Mansfield and Pleasant Hill. Becoming commander of his Cavalry brigade he was later appointed Brigadier General by Kirby Smith for his service in that campaign. With the end of the Civil War, his promotion was never confirmed by President Davis.