Economics: The economies of the North and South were interdependent, but the South was more dependent on the North for manufactured goods. This led to trade disputes and tariffs, which further strained relations between the two regions.
Slavery: The issue of slavery was the most divisive issue between the North and South. The Northern states were mostly opposed to slavery, while the Southern states were strongly in favor of it. The expansion of slavery into new territories was a major point of contention, and the abolitionist movement in the North called for the end of slavery.
Politics: The political landscape of the United States was also divided along sectional lines. The Northern states were dominated by the Republican Party, which was opposed to slavery, while the Southern states were dominated by the Democratic Party, which supported slavery. The election of Abraham Lincoln as president in 1860, who was opposed to the expansion of slavery, was the last straw for the Southern states.
In February 1861, the Confederate States of America was formed by seven Southern states that had seceded from the Union: South Carolina, Mississippi, Florida, Alabama, Georgia, Louisiana, and Texas. Four more Southern states joined the Confederacy later that year: Virginia, Arkansas, Tennessee, and North Carolina. The secession of these states led to the outbreak of the American Civil War, which lasted until 1865.