What was lincolns most likely purpose for issuing the emancipation proclamation

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.