“Slavery didn’t become the war’s focus for Northerners until later...”
The Abolitionist Movement, Bloody Kansas, John Brown, The Compromise of 1860, The Missouri Compromise, the Underground Railroad, the 3/5 Compromise, the first Republican Platform of “Free Soil and Abolition”
I tend to think people knew what the war’s focus was.
Remember, when people speak of state’s rights in this context, they’re speaking about the states right to keep and sell human beings, among other things. If we believe in Natural Law, then there are no state’s rights in the context of slavery, or the “Southern way of life.”
People knew that the political conflict was about slavery -- or rather about the expansion of slavery.
But when the war began, Northerners weren't fighting to free the slaves, but to preserve the union.
They could put up with slavery in the South, at least for the first two years of the war. Some slaveowners even supported the union and fought in the army.
A lot of the conflict has to do with the definition of words like "cause" or "reason." There are surface causes -- the reasons that people give for specific actions -- and there are deeper causes beneath the surface, which become much more apparent later on.