According to entries in his diary, John C. Crosby enlisted for military service in the Union army during the Civil War on September 9, 1861, leaving for Baltimore shortly afterward to serve in the Seventh Maine Regiment, evidently as a hospital orderly. His last entry during the period of his military service was on August 11, 1862, and by 1865 his entries indicate that he was engaged in farm work.
From the guide to the John C. Crosby U. S. Civil War Diary MS 287., 1861-1868, (Woodson Research Center, Fondren Library, Rice University, Houston, TX)