He fought with the unit at Fort Henry, Fort Donelson, Corinth, Shiloh, and Hatcher's Run. He then fell ill with typhoid fever and was discharged on August 1, 1862. He returned after recovery with the Chicago Mercantile Independent Battery Light Artillery, where he fought at Arkansas Post, Port Gibson, Champion Hill, Big Black River Bridge, Vicksburg, and Mansfield. At Mansfield, the brigade was captured and Arnold made prisoner. He was confined at Camp Ford in Texas for fourteen months.