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.