How could the USA’s westward expansion cause an intense debate over slavery?

Westward was also Southward

Explanation:

First we have to bare in mind that the Founding Fathers of the US were quite comfortable with slavery. As Alexander von Humboldt found out when he visited Thomas Jefferson at the start of the 19th century. Slavery was the only issue of dispute between the scientist and the political leader.

The westward expansion (Manifest Destiny) in the 19th century carried slavery down into the Southwest, into Mississipi, Alabama, Lousiana and finally into Texas (1840). The rapid expansion of the United States intensified the issue of slavery. So southern states that depended heavily on slavery were added to the union that ultimately didn´t proof to be very united (civil war) 20 years later.