What effect did westward expnsion have on the background leading up to the American Civil War?

The cotton-growing Southern states noted the industrial growth of the North and the tendency of Congress to pass laws favourable to North (eg, tariffs against imported goods, decision to route the transcontinental railroad via Chicago instead of New Mexico).

So the South was sensitive about the admission of new territories to the USA, and wanted at least half of them to be slave-states.

Presently this slave-empire grew rich enough to threaten a breakaway from the USA. The loss of the cotton revenues was the actual trigger of the war, with abolitionism firmly second. (Later, Lincoln turned it into a war on slavery for a mixture of reasons.)