At the start of the Civil War, Frémaux assisted in forming the Creole Guards, a volunteer company that became incorporated as Company A of the Eighth Louisiana Infantry Regiment, in which he attained the rank of captain. The regiment was sent to Manassas, Virginia, where Captain Frémaux drew maps unofficially while keeping watch over fords in the Occoquan River. There he composed Map of Manassas Junction & roads to fords on Bull Run & Occaqaan [i.e., Occoquan] River, Camp Pickens, Va., June 12th 1861,a detailed work that reveals the challenges of drawing a dependable battlefield map based on unreliable information in the months following an engagement.