Answer:
On the issue of slavery.
Step-by-step explanation:
The U.S. expansion towards the Pacific Ocean (the West) created the division between the North and the South. The reason for the divide was the practice of slavery. The South ( pro-slavery) wanted to introduce slavery as legal in the region where the North contained a lot of abolitionists who opposed slavery as an evil practice. The conflict began as settlers began to settle and buy fertile land at a fair price. Expansion towards westward lead to economic prosperity due to farming and ranches. It also fueled the manifest destiny where settlers believed they have the right to take land and spread their culture. With all this, the introduction of slavery in the region lead to tension between two sides.