President Lincoln called a meeting of his Cabinet. He warmed up with a story about Artemus Ward, then got to the issue. Antietam, and the retreat of the Confederates, had given him the victory he needed, and now he was going to proclaim Emancipation. The Southerners would have until New Year's day to rejoin the Union, otherwise, all slaves in rebelling states would be declared free:
http://www.archives.gov/exhibits/american_originals_iv/sections/transcript_preliminary_emancipation.html Interestingly, according to a New York Times editorial, this indirectly led to the acceptance of laws of war, including the Geneva Conventions:
http://opinionator.blogs.nytimes.com/2012/09/21/freedom-and-restraint/?ref=opinion