The Kansas territory was north of the 36°30′ parallel, so, per 1820's Missouri Compromise, a *free* territory (that is, a territory where slavery was prohibited).
That's the basis for Lincoln saying in this letter: "By every principle of law, ever held by any court, North or South" (as of August 1855), any Black person who's entered Kansas -- including slaves brought there by their owners -- is free.
But proslavery forces wanted to change that. https://en.wikipedia.org/wiki/Bleeding_Kansas
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