Under the laws of that US government, the slaves had no say. Given that the United States government didn't give a sh*t about them for "Four Score and Seven Years", I don't see why business as usual would have suddenly been regarded as a crises.
Certainly the USA didn't launch that war against the South for the benefit of the slaves, though it is they who ultimately benefited from it most.
As Charles Dickens so eloquently put it:
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."
Had the New York/Washington DC power axis won the war more quickly, the Slaves would have continued their toil in the fields, and the money going into New York and Washington pockets would have continued as before, though they would have probably wanted a bigger cut than previously.
Totally false, but what is true is that Dickens himself hated the North and loved the South.
So Dickens' report on the Civil War should no more be taken as gospel than, say, a Nancy Pelosi report on "Dreamer" legislation.