Chapters 15-18 explore the Civil War and Reconstruction era, discussing key events such as the battles of Gettysburg and Vicksburg, the Radical Republican reconstruction plans, and the rise of industrialization during the Gilded Age. The text also highlights influential figures like Andrew Carnegie, Ulysses S. Grant, and William 'Boss' Tweed, as well as the formation of labor organizations such as the Knights of Labor. Additionally, it addresses issues related to freedom, imperialism, and social reforms leading into the Progressive Era.