The declaration, of and by itself did not abolish slavery but attempted to free only the slaves in those states at war with the US. Slavery was acceptable among friends, just bad for enemies. The proclamation was only as good as the bayonets that enforced it.
It was not congressional policy but an executive military decree. Legal? No. It was confiscation of private property without appeal or compensation and without congressional or judicial approval. It was this illegal proclamation that made Lincoln known as the Great Emancipator.
His masterpiece has been called our finest document since the Declaration of Independence. It has also been called a masterful military strategy. It has been called a devious way to lure slaves into a bloody revolt. That last idea is just wrong. Lincoln did not want a slave rebellion. If the slaves had started killing white women and children in an uprising, the Yankee soldiers would have come to the rescue of the whites, orders or none.
The proclamation was not humanitarian and not entirely military. It was a very shrewd political document. It turned the war into a crusade. The problem was that the men and boys on both sides, who were wounded and killed had no dog in the fight. Instead of Billy Yank being home raising hay and Johnny Reb being home raising cotton, they were on the battlefields raising hell.
That war period is probably the dirtiest period in our history.