Civil war revisionism is generally thought of as a right-wing thing. It's generally the former cnfederate states in the south that want to claim that it wasn't actually about slavery. But I grew up learning a liberal brand of revisionism, making the claim that the war wasn't fought over slavery. Basically that claim goes like this:
...while there were states without slaves, no part of America cared enough about emancipation to fight a war over it. The idea that the civil war was fought to end slavery is itself modern revisionism, that tries to make early America look more humanitarian, and obscure the actual brutality of slavery, and indifference to it.
This point of view doesn't seem so prevalent now as it did in my childhood, so I've left it behind. Still, it does seem extraordinary that Lincoln found the support to go to war, essentially over slavery. Even in the North, racism still exists in significant ways today; back then, even in the north, it must've been a pretty mainstream opinion that African people were less than human. So It's my knee-jerk reaction to believe anyone who says there wasn't the support for such drastic action as a war to end slavery.
Or am I being too cynical? Is it the plain and simple truth that when the South threatened to secede over slavery, the North was so against the institution that the fought against the establishment of an independent slave nation, and fought to maintain a specifically slave-free union. Or were there enough objections for war amongst Northerners that other justifications had to be sought?
*On a side note: since Trump's election, there's been half-serious talk of liberal states like New York and California seceding. I read an article that equated this to Union leaving the confederate to its own devices. So goes that argument, if blue states seceded, millions of Americans in red states would still suffer under Trump, and the blue states would only be saving themselves. Likewise, had the war not been fought, the north would've left millions of Americans enslaved, saving only itself from the evils of slavery. This is, of course, how I prefer to think of the war - a more perfect union, protected in order to pursue emancipation for every American. But it still rings hollow.
Just because slavery was the core of the reason as to why Southern states attempted to leave the Union does not imply that soldiers on either side fought to abolish or preserve slavery. Southern state governments put a spotlight on slavery as an important issue. Many Southerners probably fought with this in mind, but motivations were many.
That said, preserving the Union was the rallying cry for many in the North. Again, certainly there were those who fought to put an end to slavery and individual motivations were certainly diverse and complex. Still, preserving the Union was the first order of business, because if states could leave whenever they wanted over whatever issue seemed important at the moment, the Union would certainly have dissolved long ago.