At the outbreak of the Civil War, Massachusetts native Gilbert Thompson joined the regular army, which assigned him to the engineer battalion, a unit that provided critical support for the Union military effort in building bridges and roads and surveying and producing maps. While serving, Thompson kept a journal that eventually filled three volumes. The author's early education in a utopian community called Hopedale left him well read, affording a journal peppered with literary allusions. Once the war ended, Corporal Thompson added some postwar reflections to create a unified single volume, which editor Mark A. Smith has carefully arranged so that the reader can clearly distinguish between Thompson's contemporary accounts and his postwar reminiscences. An accomplished artist and topographer, Thompson illustrated his journals, adding depth to his narrative with portraits of key figures, drawings of ordinary scenes such as soldiers playing chess, and sights of the war. Additionally, he collected photographs both during and after the war, many of which are included.

Thompson's wartime musings and postwar recollections have much to offer. Few diaries contain glimpses into the workings of a highly specialized unit such as the engineer battalion, and Thompson's skills in depicting daily camp life in both words and pictures provide a distinctive look at the Union Army during the Civil War as well as an insightful look into the human condition. In his 1879 introduction, Thompson writes, "I wonder how I wrote as much and as well, and am thankful I was so fortunate as to have the opportunity to do so." Students of the Civil War will feel fortunate he did.