As the united states expanded in the 1800s, the north and south disagreed about whether

Slavery should be allowed

Explanation:

As the US expanded its territory the question about whether slavery should be allowed in the new territories started to appear. The Anti-Slavery states and Abolitionists wanted the new territories to be free states so the balance between pro-slavery and anti-slavery was maintained.

At the same time, Southerners fought to assert their rights and expand slavery. A series of Treaties were proposed and in the end, the new territories were able to decide if they would be a free slave or not

they united the colonies in their fight against the british.