Milton Barnes was a recently married lawyer in Cambridge, Ohio, when the Civil War began. He served first as a captain, commanding G Company of the 62nd Ohio Volunteer Infantry Regiment. This unit participated in the first Federal invasion of the Shenandoah Valley in the spring of 1862. When the 62nd Ohio was transferred to the Rappahannock in mid-1862, Captain Barnes was discharged on medical grounds and returned briefly to Cambridge, Ohio.

Later in 1862 he obtained an appointment as lieutenant colonel of the 97th Ohio Volunteer Infantry Regiment and served as its deputy commander and often-acting commander until mustered out in June 1865. Lieutenant Colonel Barnes fought with the 97th Ohio in major engagements in Kentucky, Tennessee, and Georgia until wounded at Kennesaw Mountain, Georgia, in June 1864. He returned to the field in time to take part in the defense of Tennessee against the Confederate re-invasion and was again wounded at Nashville, Tennessee in December 1864. The remainder of his and the 97th Ohio's service was on provost duty at Huntsville, Alabama, and in the East Tennessee mountains on guard against a last possible threat from the failing Confederates in Virginia.

His war letters include first-hand accounts of several battles, notably Stones River (Murfreesboro) and Missionary Ridge at Chattanooga, as well as, details of ordinary life on campaign and in camp. The occasional gaps in the correspondence were mostly the result of several campaigning which left no time for writing. His wife's letters include political news and comments as well as family matters.

After the war, Barnes resumed law practice in Cambridge. In 1867 he was brevetted Colonel in the Volunteer Force, AUS, to date from March 13, 1865 for "gallant and meritorious service" in the war. He was elected Secretary of State for Ohio for two terms (1877-1881) and later served for a year (1891-1892) as special Treasury Agent for Alaska. A lengthy account of his arrival and duties at St. George Island in the Bering Sea is included in the collection.