During the late 1850's, the North and South were vastly different places.
In the North, immigrants were reshaping the landscape in large east coast cities and the midwest. There was more than twice the amount of miles of railroad laid in the north as the south and it was much easier for people and goods to move from state to state.
Cotton and tobacco were the dominate industries the South. With slave wages at no more than the cost of keeping a human being alive with basic food, water and shelter, the concentrated aristocracy in the south felt no need to join the industrial and transportation revolution in the north.
Slaves begat more slaves and plantation owners would pour the profits back into expanding their estates, which would produce more capital, which could buy more slaves. They primarily exported raw goods and imported luxury or finished goods from Great Britain since with the sparse tracks of railroad in the south, it was easier than buying the products domestically.
Eventually, all the good land in the old south was used up, and enterprising southerners bought up westward lands from the federal government and looked to expand their agricultural feudal system.
In the west, northerners and southerners would mingle, both seeking to dominate the territories they were settling. With the new 'religious enthusiasm' in the north as Lincoln called it, slavery became the great moral issue of the day in the north and west.
The westerners from the north brought a new urgency to the anti slave movement and the south felt threatened.
As the north entered into the industrial revolution, new paths to social mobility were open even for immigrants while class division and slave driven feudalism continued to dominate the south.
The south did have one large advantage it would use in the war though. It had most of the military colleges and by far the most experienced generals, as the military culture fit in much better with the south's aristocratic world view.
This is where the two sides stood before the war. The north and south had dramatically different economies, world views, and infrastructure and would fight to the death to preserve them. The western states would become battlegrounds both in the literal sense and for the hearts and minds of their inhabitants, with massive amounts of money and effort spent on recruitment and propaganda in all the border states by both sides in the war.
This is the world Lincoln stepped into in 1861, with 8 years of almost criminal neglect of the deteriorating situation by Buchanan and Pierce. As a border state president from Illinois, he would be on the front line of the hearts and minds war.