Robert A. Duncan enlisted as a second lieutenant in (old) Co. A, 13th Arkansas Infantry, at Harrisburg, Arkansas, on July 29, 1861, for twelve months. He was wounded at Shiloh, Tennessee, on April 7, 1862. He reenlisted for three years at Tullahoma, Tennessee, on April 28, 1862. He was elected major in the regimental reorganization following the battle of Shiloh; was promoted to lieutenant-colonel; and resigned and was dropped from the rolls on February 2, 1864.