American Civil War: population of the Union states 1860-1870

Prior to the American Civil War, New York, Pennsylvania, and Ohio were the most populous states in the Union, each with between two and four million inhabitants. Industrialization in the north was one of the key drivers of population growth during this period, through both internal and external migration, and Illinois saw the largest population growth during the 1860s largely due to the expansion of industry around Chicago. The gradual industrialization of the north in the early 1800s also contributed to the decline of slavery in the Union states, and the economic differences between the Union and Confederacy was a key factor in both the build-up to the Civil War, as well as the Union's eventual victory in 1865.