That is an excessively broad statement. While that statement is likely true, it misleads greatly as to the primary cause of the war.
What I have learned is that the South was producing 73-85% of all export value for the United States. The Northern states only produced 15-27% of the total export value to Europe, but strangely enough, 90% of all the money for the entire US export production funneled it's way back through New York and Boston.
The Northern Coalition had rigged the game so that virtually all the slave money flowed through their hands. They set the tariffs high because that boosted their profits for domestic manufactured goods, and the laws were rigged to give New York virtually total control on all shipping and imports. It is estimated from various sources that New York was getting 40% of the total profits from slave produced goods, and Washington DC collected their tax percentage on these same goods, and that absorbed another 20-30% of the entire value of all those slave produced goods.
New York and Washington DC were making a fortune off of Slavery, (More than the people actually running the plantations) and they didn't have to do much of anything to collect that money.
When the South decided to become independent, that flow of money (230 million per year in 1860 dollars) was going to stop. Worse, European goods would flow into the Southern states at greatly reduced prices, and there displace goods which were previously manufactured in the North, costing even further financial losses to the North.
Worse still, the Southern states would began supplying European manufactured goods all along the watershed of the Mississippi river, destroying the Midwestern market for goods manufactured in the North East.
Worse still, as the border states realized they could make more money off of the CSA system than they could off of the USA system, they would have eventually moved to join the Confederacy, causing even further losses of money and power to the then greatly weakened USA.
Where does slavery come into all of this? The slaves were producing the money. The fight was over the money, not over freedom for the slaves.
The reason Lincoln urged passage of the Corwin amendment was because they wanted those slaves to remain in the status quo of producing all that money which was flowing through New York and Washington DC hands!
The sticking point of the war was independence. There was no sticking point about keeping slaves in slavery. Both sides had intended to do that when the war began. The North continued keeping slaves for 8 more months after the war ended.
That's why I say that people have been misled about the cause of the war. Slavery is only indirectly involved. The real cause of the war was a fight over money.
For a view of the situation from the perspective of a British Abolitionist, Charles Dickens had this to say about the matter.
I take the facts of the American quarrel to stand thus. Slavery has in reality nothing on earth to do with it, in any kind of association with any generous or chivalrous sentiment on the part of the North. But the North having gradually got to itself the making of the laws and the settlement of the tariffs, and having taxed South most abominably for its own advantage, began to see, as the country grew, that unless it advocated the laying down of a geographical line beyond which slavery should not extend, the South would necessarily to recover it's old political power, and be able to help itself a little in the adjustment of the commercial affairs.Every reasonable creature may know, if willing, that the North hates the Negro, and until it was convenient to make a pretense that sympathy with him was the cause of the War, it hated the Abolitionists and derided them up hill and down dale. For the rest, there's not a pins difference between the two parties. They will both rant and lie and fight until they come to a compromise; and the slave may be thrown into that compromise or thrown out, just as it happens."