Later that year, as Ambrose E. Burnside marched toward Knoxville, Tennessee, he bypassed the Cumberland Gap, leaving a single brigade posted there. Burnside occupied Knoxville on September 1, 1863 and turned his attention back to the Cumberland Gap, dispatching Shackelford's brigade. Shackelford demanded that Confederate general John W. Frazer surrender the gap, but Frazer refused. Burnside personally arrived with reinforcements and forced the Confederates to surrender. Later in the month, Shackelford was promoted to command a division in the XXIII Corps and then in November to command of the Cavalry Corps in the Army of the Ohio. Shackelford's cavalry helped to secure the vital cross roads at the Battle of Campbell's Station and participated in the Siege of Knoxville. When James Longstreet's Confederate forces retreated from Knoxville, Shackelford was sent in pursuit. He caught up with the Confederates, but in the Battle of Bean's Station Longstreet checked the Union pursuit before withdrawing from the region.