This dissertation explores the combat performance of the Union soldiers who filled newly-raised regiments that fought through the Civil War’s final year. Period observers and historians have typically regarded such later arrivals as substandard to the “Boys of ‘61” who enlisted at the war’s start. Tapping the methods of social and traditional military history, this work is among the first to assess the record of these soldiers under fire. It does so by tracing the experiences of the 17th Vermont and 31st Maine Volunteer Infantry Regiments, starting with their formation and continuing with their service throughout the Overland and Petersburg Campaigns (May 1864 – April 1865). Both outfits fought in the Army of the Potomac, the Union’s largest field army, in which only half of whose veterans reenlisted on the expiration of their original three-year terms. The 17th and 31st maintained moderate to high levels of unit cohesion, showed determination to accomplish battlefield objectives, and sustained heavy casualties in the process. This project justifies a reappraisal of the later arrivals, a population of approximately 820,000 white men who donned the uniform between 1863 and 1865. These forgotten boys in blue left behind a record of valor and sacrifice essential to achieving the destruction of the Confederacy. / History