In 1861 the deadly Civil War broke out. The North and the South each fought for the maintenance of their respective ways of life. For the South it was the preservation of slavery to fuel their economy, and for the North it was the preservation of the United States. At a time when the Northern troops were getting clobbered and losing motivation, President Abraham Lincoln signed the Emancipation Proclamation that freed all slaves in the rebelling southern states and gave the Union a much-needed motive for fighting that turned the tide of the war and resulted in a Union victory.
Moore, Zachary W., "The Emancipation Proclamation" (2012). U.S. Experience to 1877 Podcasts. Paper 4.