Isaac Barnum, like his brother George and father Levi, served in Company "F", 56th NY Infantry. He enlisted 16 Aug 1861 at Liberty, to serve 3 years, and mustered in as a Private in Company "F" 29 Aug 1861. He was discharged for disability 16 Oct 1862, at Fort Monroe, Virginia.
From records found in the NY State Archives, Albany, NY. NY State Adjutant General's Office Registers, Vol. 25: Isaac Barnum (1840/1-1864) died on 15 Jul of wounds received on June 17, 1864 at Lynchburg VA. He was enlisted twice. First, mustered in as a private to the 56th Infantry, Co F on 29 Aug 1861; he was discharged for disability on 16 Oct 1862 at Ft. Monroe, FL. He then reenlisted and was part of the 15th NY Cavalry, Co E, Jan 1864- July 1864.