The Civil War: a Comprehensive TextbookAmerica's evolution during the mid 19th century just prior to the Civil War was amazing. The six fold increase in population from 1800 to 1860 was fueled by an influx of immigrants into the North and Northcentral, and the 800,000 immigrants into the North between 1861 65 alone filled the Union armies. This population explosion coincided with a dramatic territorial expansion. From 1845 48, the United States grew by over 1,200,000 square miles, a 67%