According the History Editors at 1862 the President Abraham Lincoln issues an early emancipation proclamation which let the freedom around of 3 million black slaves in United States of America.
Mr. President Abraham defended the position or argument that the war ( at 1861) was about restoring the Union and not about slavery.
In other words probably the main reason for issuing the emancipation was to establishment of black military which now they had a condition of freedom not of slaves.