The primary cause of the American Civil War was the contentious issue of slavery and its expansion into new territories. As the nation grew and new states joined the Union, debates intensified over whether these areas would permit slavery. The economic and ideological divisions between the industrial North, which favored free labor, and the agrarian South, which depended on slave labor, created deep-seated tensions. The election of Abraham Lincoln in 1860, viewed by many Southern states as a threat to the institution of slavery, led to the secession of several Southern states and the formation of the Confederacy. The war resulted in significant loss of life and ultimately led to the abolition of slavery, shaping the future of the United States.