In the decade leading up to the Election of 1860 and the subsequent start of the Civil War, regional tension increased due to multiple factors. In your initial post, describe one key event from the 1850s that escalated tensions between the North and South. In your description, consider the arguments for both sides. How did the push for western expansion impact the event you described?

One key pair of events occurred in Kansas in 1856: the sacking of the town of Lawrence, a center of the free-soil movement, by pro-slavery forces, followed several days later by the retaliatory strike of John Brown and his men, in which five pro-slavery settlers were killed. Both of these incidents took place as a result of tensions that already existed in the realm of westward expansion between free-soil and pro-slavery settlers.
The question of how to deal with the western territories more directly affected the fragile status of the Union in the 1850s than did the fact of slavery being practiced (without any end in sight) in the actual states of the US. Many Americans who were not in favor of outright abolition were still against allowing slavery to spread to the territories, the regions in the West which had not yet been organized and admitted to the Union as states. This was the position of Abraham Lincoln and most of the newly-formed Republican party: that they didn't wish to interfere with slavery in the states where it was already in place, but that there should be absolutely no extension of it to the newly settled territories. Even before the sacking and looting of Lawrence, Kansas and Brown's Pottawatomie "Massacre," a low-level guerrilla war had been taking place in "bleeding Kansas." The Southern states which were eventually to secede in 1860–1861 wanted to make the establishment of slavery in the territories a fait accompli by encouraging the westward drive of slave owners, believing that if enough of them settled in the West, it would be impossible to eradicate slavery there—just as they believed it could not be eradicated in their own states, since a majority of Northerners did not have the desire to interfere with it there.
The Lawrence and Pottawatomie events served to exacerbate tensions further. John Brown became a hated figure in the South, three years before his his raid on the arsenal at Harpers Ferry, VA, and his attempt to foment an uprising of enslaved people. In the Northern view, the pro-slavery "bushwhackers" of Kansas became equally notorious and hated for their ruthlessness and their attempts to impose their will on the free-soil settlers. It was only a matter of time before all of this came to a head with the election of Lincoln in 1860 and the secession of the Southern states and the war that followed.