On October 21, 1861, Simms was appointed to the temporary rank of colonel in the Confederate States Army. He was appointed lieutenant colonel in the Provisional Army of the Confederate States on December 24, 1861, and was assigned to the First Battalion, Kentucky Cavalry.
Simms' military career took a significant step forward when he was promoted to a temporary colonel in the Confederate States Army on October 21, 1861. He later received a permanent appointment as lieutenant colonel in the Provisional Army of the Confederate States on December 24, 1861, and was assigned to lead the First Battalion, Kentucky Cavalry.