The primary cause of the American Civil War was the contentious issue of slavery, particularly its expansion into newly acquired territories and states. The southern states, reliant on an agrarian economy and slave labor, sought to protect the institution of slavery, while the northern states, increasingly industrialized and favoring free labor, opposed its expansion. The election of Abraham Lincoln in 1860, who was viewed as an opponent of slavery's spread, further exacerbated tensions, leading to the secession of Confederate states and ultimately the outbreak of war in 1861.