"The [Emancipation] proclamation has no constitutional or legal justification except as a war measure." A. Lincoln Letter to Sec. of Treas. Salmon P. Chase; 3 Sep 1863

Any informed scholar knows that it was a propaganda tool intended to (hopefully) cause widespread slave uprisings to drain Confederate soldiers to quell them (none occurred); to
create a "cause" around which the North would rally (200,000 Union soldiers immediately deserted); and to prevent seemingly-close diplomatic recognition by France and the British Empire (which would have meant fighting those navies to maintain the blockade). 

Since Lincoln knowingly excluded freeing a single slave in any area of the North and South controlled by the Union Army, it was a transparent lie.