Lincoln wanted to keep Europeans from helping the South. The incentive for them to help was Europe needed the South's agricultural products.

Making it "about slavery" he knew would appeal to European intellectuals.
Hence the Emancipation Proclamation" which applied only to states that were rebelling.

Quote:

Emancipation Proclamation summary: The Emancipation Proclamation was issued by President Abraham Lincoln on January 1, 1863, as the country entered the third year of the Civil War. It declared that "all persons held as slaves shall be then, thenceforward, and forever free"but it applied only to states designated as being in rebellion, not to the slave-holding border states of Delaware, Kentucky, Maryland, and Missouri or to areas of the Confederacy that had already come under Union control. The careful planning of this document, with Lincoln releasing it at just the right moment in the war, ensured that it had a great positive impact on the Union efforts and redefined the purpose of the war.

http://www.historynet.com/emancipation-p....

Great strategic move.

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