Yeah, the Southern side is pretty straightforward. They were literally fighting for their way of life.
The Northern side was much more complex, and wasn’t as anti-slavery as modern history portrays it.
In the North, there had been an abolitionist movement since before the U.S. was even a country. The Quakers were abolitionists, and some other religious groups also believed in abolition. The abolitionist movement had been growing all through the 1800s, but by the time of the Civil War the entire North was not abolitionist.
The other major political force in the North was the industrialists. They saw the agricultural way of life as on its way out, and industrialism was the way that the world was going (and to be fair, they were right). They felt that the Southern plantations were holding back progress and choking northern industrial companies, but then what was good for the South was bad for the North and vice-versa. If you put in protective tariffs so that manufactured goods from Europe were more expensive than locally made goods, then you protected the new northern factories. But then European countries would enact tariffs in response, which would hurt the sale of cotton and tobacco and screw over the southern plantations. Lower the tariffs, and the plantations thrive but the factories can’t compete with the European goods, and the factories starve.
So now you have the old “the enemy of my enemy is my friend” situation, and the abolitionists and the industrialists joined forces to create one big political party that was both pro-industry and anti-slavery.
But the one thing you have to keep in mind is that about half of this anti-slavery party really didn’t give two hoots about slavery, and about half of this pro-industry party really didn’t give two hoots about industry.
If you read Lincoln’s speeches from the time, you can see which half of the party he was addressing. If he was talking to industrialists, he would minimize the anti-slavery rhetoric and would talk up things like protection for northern industry. If he was talking to abolitionists, he would crank up the anti-slavery topics and minimize the industrial talk.
And from the Southern point of view, it didn’t matter that the Republican Party had two major groups in it. They saw one big enemy party that was both anti-slavery and anti-plantation.
So why did the North actually go to war?
Part of it is because they were attacked. But a major part of it was because of Lincoln and the Republican Party. There was actually a lot of debate about whether the South should be allowed to secede and just let them go their own way. A lot of folks in the North were in favor of it, or at the least they didn’t care too much one way or the other. But Lincoln and the Republicans were pro-Union, and they were the folks in charge.
The North really didn’t expect the South to put up that much of a fight. They thought the war would be over in a couple of weeks, so that probably factored into their decision-making as well. And to be fair, the South thought the same thing. They didn’t think that the North had the stomach for a protracted war. The South didn’t have to defeat the North. All they had to do was make the North give up, and once the fighting got down and dirty, the South was confident that the North would back off.
It didn’t work out the way that either side predicted.