It fought to preserve a union of legalized slavery, and it won, then illegally abolished slavery to stop the economic power of the South from threatening the New York/Washington DC power cartel that has been controlling the nation ever since.
But when it started the fight, it was planning to keep the slavery.
Abolition would have eventually become the goal of the Union and that would be irresistible. With enough added states there could have been a Constitutional Amendment outlawing it.