.Sure, Blacks were fighting for emancipation, but the North was primarily concerned with preserving the Union, and slavery did not fit into the view of "One Nation under God". This historical record of the views from the South is revealing...
In March 1861, Alexander Stephens, vice president of the Confederate States of America, gave his view:
The new [Confederate] constitution has put at rest, forever, all the agitating questions relating to our peculiar institution — African slavery as it exists amongst us — the proper status of the negro in our form of civilization. This was the immediate cause of the late rupture and present revolution . . . The prevailing ideas entertained by . . . most of the leading statesmen at the time of the formation of the old constitution, were that the enslavement of the African was violation of the laws of nature; that it was wrong in principle, socially, morally, and politically. . . Those ideas, however, were fundamentally wrong. They rested upon the assumption of . . . the equality of races. This was an error . . .
Our new government is founded upon exactly the opposite idea; its foundations are laid, its corner–stone rests upon the great truth, that the negro is not equal to the white man; that slavery — subordination to the superior race — is his natural and normal condition.
— Alexander H. Stephens, March 21, 1861, reported in the Savannah Republican