Soon after the beginning of the War with Fort Sumter, Casement volunteered for ninety days militia service with the Seventh Ohio Volunteer Infantry. He was also elected major of the regiment. Following his ninety days, he re-enlisted for three years. Casement's first battle was at Kessler's Cross Lanes,Virginia on August 26, 1861. Although the Union forces were surprised and routed with severe loss, Casement, at the head of the Union left wing, commenced a retrograde march through Confederate territory over mountain ranges and rivers to Charleston, West Virginia without the capture of a man. Casement also served in the Shenandoah Valley against Confederates under Stonewall Jackson. After his heroic actions at the First Battle of Kernstown near Winchester Virginia, March 23, 1862, Casement was appointed colonel of the newly formed 103rd Ohio Volunteer Infantry and fought in the operations around Knoxville.