The addition of territory in the frontier lands, and the subjugation of the Native tribes there opened up vast new territories for settlement and agriculture.
While sectional tension between slave-based economies and those of the North had been growing ever since the Constitution, the addition of these new lands threw gasoline on the fire. The omnipresent question in the legislature and economic circles became "Would the new territories have legal slavery or be designated as free"?
As notable figures like Senator Henry Clay successfully compromised on the slave-free debate with the Missouri Compromise of 1820 and the Compromise of 1850, along with the Fugitive Slave Act, the amount of sectional tension grew in proportion to the number of slaves and slave territories.
Historians often argue that the Civil War was inevitable at some point, but many historians also argue that the reason it was in the early 1860s was because westward expansion forced the issue.