Sheldon M. Novick, "Holmes, Oliver Wendell," American National Biography Online, February 2000, http://www.anb.org/articles/11/11-00423.html.
After the outbreak of the Civil War in the spring of 1861, Holmes enlisted in the Massachusetts militia, eventually obtaining a commission as a lieutenant. He served for two years in the Twentieth Massachusetts Volunteer Infantry and fought at Ball's Bluff, the Peninsula campaign, and Antietam. In those first two years he was promoted to captain, was wounded three times, twice nearly fatally, and suffered from dysentery. Exhausted, he completed the third and final year of his enlistment, in the winter of 1863-1864, as aide to General Horatio G. Wright and then to General John Sedgwick of the Sixth Corps. In the relative leisure of winter quarters, Holmes turned to philosophical writing, developing from his combat experience a purely materialist evolutionism. History was shaped by the perpetual conflict of rival nations and races, he believed. Laws were written and governments established by the victors.