Two days before Lincoln's inauguration as the 16th President, Congress, consisting only of the Northern states, passed overwhelmingly on March 2, 1861, the Corwin Amendment that gave constitutional protection to slavery. Lincoln endorsed the amendment in his inaugural address, saying "I have no objection to its being made express and irrevocable."
Quite clearly, the North was not prepared to go to war in order to end slavery when on the very eve of war the US Congress and incoming president were in the process of making it unconstitutional to abolish slavery.
Here we have absolute total proof that the North wanted the South kept in the Union far more than the North wanted to abolish slavery.
If the South's real concern was maintaining slavery, the South would not have turned down the constitutional protection of slavery offered them on a silver platter by Congress and the President. Clearly, for the South also the issue was not slavery.
The real issue between North and South could not be reconciled on the basis of accommodating slavery. The real issue was economic as DiLorenzo, Charles Beard and other historians have documented. The North offered to preserve slavery irrevocably, but the North did not offer to give up the high tariffs and economic policies that the South saw as inimical to its interests.