Lincoln’s version of preserving the union was all about maintaining the flow of money to keep the federal government in a position of power. After all, the South provided the funding via tariffs for 80% of the Federal budget. Without the South, the Federal government would be insolvent. He did not run for office on the premise of freeing the slaves. He did say he wanted to insure any new territory that was accepted for statehood would not be a slave bearing state. This meant any slave owner could not move from a slave state to a free state and take his slaves with him. So slavery was a part of it, but the main driver was the South was tired of being held in bondage being the primary funder to a Federal government from which they received little to no benefit. The Emancipation Proclamation is a very well written, very short document that was not written until two years after the start of the War. Union losses up to that point were significant and Lincoln was on the road to failure and a likely failed re-election. He needed a rallying call to market himself and justify what had become an unpopular war. Slavery fit the bill.