The United States emerged from the war as a world military and industrial leader. Unlike the war-torn cities of Europe the homes and industries of the nation were relatively unscathed by the Great War. The late entry of the US meant that fewer men had lost their lives than in Europe
For the answer to the question above, this refers to Three-Fifth Compromise where the South used the number of slaves in its territories to increase its representation. It gave the South more power by using the slave population to its advantage. This would threaten the balance of power, particularly to the Northern States.
Unfair working conditions and child labor and eventually brought about worker unions
