The Emancipation Proclamation.
It could not carry authority in those states, of course (although it licensed Union troops to free any slaves they found), but it did have the effect of preventing Britain from sending military aid to the Confederates. Britain had abolished slavery thirty years earlier, and could not be seen to fight for the slave-owners.
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It prohibited slavery North of a certain parallel, but only in the territories brought in under the Louisiana Purchase. When the new Mexican territories came in, they needed a new compromise. That one did not hold.
The Missouri Compromise, which was passed in 1820, was a document that involved the regulation of slavery. To do this, slavery was prohibited in the former Louisiana Territory (north of 36°30′N), with Missouri being exempt.The balance between the number of "slave states" and "free states" was achieved by creating a free state (Maine) out of the northern region of what was then Massachusetts.
The Missouri Compromise addressed slavery in the Arkansas and unorganized territory of the Great Plains. Slavery was prohibited in all of these areas, except within the boundaries of Missouri.
Massachusetts prohibited slavery in 1780
The parallel of Missouri's Southern border was the official reference-point for the new Western territories when they became states of the union. Anywhere north of that line - all the way to the Pacific - slavery was illegal.