What is your point?
The reason the American Civil War was being fought was inexorably tied to slavery...no matter what Abraham Lincoln said in that letter.
Well, if you notice, what he said in that letter involved slavery in every option listed because the war was about slavery. And ultimately what he found (and ultimately was successful in) was that freeing the slaves could save the Union, not some mixture, not just keeping the status quo, only one of those options turned out to be the one that worked.