The main reason for the problem between the North and South in the United States leading up to the Civil War was primarily slavery. While factors like farming, economy, and industry also played significant roles in the differences between the two regions, the fundamental conflict centered around the institution of slavery and whether it should be allowed to expand into new territories and states.
The Northern states, which were increasingly industrialized and had a growing economy that relied less on slave labor, tended to oppose the expansion of slavery. In contrast, the Southern states were agrarian and dependent on slavery for their agricultural economy, particularly in the production of crops like cotton. This fundamental disagreement over the morality, legality, and economic implications of slavery was at the heart of the sectional tensions that ultimately led to the Civil War.