Reading The Abolition Of Slavery (you can get this free ebook in Mobipocket format) revealed an interesting fact about US history, concerning the procedure of slaves emansipation. This procedure was difficult to perform without facing a contradiction with existing legislation (as the Congress initially refused to interfere with the institution of slavery).
The solution was found in putting this civil problem under the war jurisdiction, as Congress had limited war authority and the powers of war were limitated by the laws of the nations only. And according to the war powers slaves belonging to an enemy were regarded as belligerents first of all (for they had supported enemy's economy with their labour).
So, Southern slaves were being first taken as captives and then set free as a prisoners of war. Released captive is a free man, so it resulted to slaves in loosing their previous status. More exactly, not loosing but becoming free people in addition, which was enough to prevent masters from reclaiming their property back.
What has drawn my attention in this story is how it was necessary to become a prisoner first in order to become free.