I was just reading Richard Dawkins’ new book “Outgrowing God” and I came across the answer to your question.

In 1858 Lincoln wrote” I will say then that I am not, nor ever have been, in favor of bringing out in anyway the social and political equality of the white and black races”

In fact during the early days of the civil war the Union army would return runaway and found slaves to their southern masters. After their first relative success of a tactical tie at Antietam (really the first “good” result for the Army of the Potomac) lincoln used the “Emancipation Proclamation more as a demonstrative device to show that the tide of the war has turned. Though it did free slaves in conflict districts it also was a morale boost for the Union supporters who were in desperate need of good morale.