The settlement of the west increased tension between the North and South due to the issue of slavery. The new territories that the US were adding in the western part of North America had to determine whether or not slavery would exist. To do this peacefully, Congress passed different laws to solve the problem like the Missouri Compromise and the Compromise of 1850.
However, none of these laws worked well. The last straw of this tension between the North and South was the Kansas-Nebraska Act. This allowed citizens of these states to vote on whether or not they wanted slavery. This law resulted in voter fraud and violence and proved that America was a country that was either going to be all free states or all slave states.