Sectionalism was one of the most significant causes of the Civil War. During the 1800's the North and South went in two very different directions both politically and economically. Politically, the northern states started to slowly but surely abolish slavery (starting with Vermont in 1777). The south however, refused to pass any laws that got rid of slavery. This was due to the fact that the south relied heavily on slave labor on farms and plantations.
The structure of the North and South's economy was another significant difference. The North was built around shipping, manufacturing, fishing, and trading due to their climate and geography. The North started to get away from group production (compared to their approach during the 1700's) because of the short growing season. The South on the other hand, focused almost solely on farming cash crops like tobacco and cotton. Due to their favorable climate and mild winters, southern states relied heavily on farming to make their economy grow.
These growing differences on how the country should be run help to cause the Civil War.