Accounts of the build-up to the American Civil War put a lot of weight on the concept of balance between the number of free states and slave states in the union, and various complicated wrangling that was necessary to preserve parity between them, deciding whether newly organized states would be "admitted as slave states" or "admitted as free states".

If I understand correctly, whether slavery was allowed in a particular state was not up to the state legislature but decided centrally by Congress. This was somehow tied to a wish to maintain a strict balance between proponents and opponents of slavery in the federal senate. Apparently there seems to have been an assumption that a senator elected in a state where Congress had allowed slavery would (must?) be a proponent of slavery, and a senator elected in a state where Congress had forbidden it would (must?) oppose it.

Was there any mechanism to ensure that each senator's views matched the particular rules that Congress had decided on for the state they represented?

It seems to me that the idea that the federal institutions rather than states themselves decided where slavery would be legal, would have been illusory as long as Congress was in complete balance. If, say, the population of a free state wanted to become a slave state, or vice versa, all they had to do would be to elect one senator who supported the new status, and there would then be a majority in the senate for changing the slave/free assignment of the state anyway.

So why did politicians care, when admitting a new state, whether initially to classify it as a slave state or a free state, if the population of that new state would in practical terms have the power to undo that decision anyway?

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    It was decided by state legislatures, except that in Massachusetts it was decided by a court ruling in 1782 and in Vermont by a provision of the 1777 constitution, and there may be similar exceptions, but in all cases it was state rather than federal authorities. Apr 17, 2017 at 19:45
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    Short answer? Not very well.
    – Schwern
    May 30, 2017 at 2:30

3 Answers 3


The way it actually worked in practice is that when Congress set up territories, it would designate them as Free Soil or Slave Territories. Early in the nation, the area above the Ohio River was so designated as Free, while KY and Tennessee were slave territories. This managed to extend the dividing line of Free/Slave more or less westward and controversy was at a minimum. Slaveholders would move to a slave Territory, non-slaveholders to a free one, and when the state was set up there was little surprise to anyone. The residents would set up the constitution of the new state to perpetuate the existing arrangement.

This worked out well enough until two things happened - first the gain of considerable non-slaveholding area from Mexico led to some calls to keep this going (the Wilmot Proviso). The other destabilizing matter was that the land in this new SouthWest was not suited for high manpower farming via slaves anyway. With no room for expansion, the South started to think of itself as 'encircled'. The American universal equation of lack of territorial growth with stagnation hurt as well.

  • Kentucky was never a territory: It was part of Virginia until the moment in June 1792 when it became a separate state. Apr 17, 2017 at 19:41
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    Actually, the Wilmot Proviso was a call to keep "this" from going. It was never ratified, but was often argued over. It was a ban on slavery in any land gained from Mexico in the Mexican war.
    – Bill Nace
    May 31, 2017 at 3:01

This isn't quite right. Slavery was considered a commercial matter, and thus the only part of it Congress felt they could regulate was its interstate commerce aspect. IOW, slave trading. This is why the Constitution specifically forbid Congress banning slave trading until 1808. That was all the danger they felt Congress could pose directly to slavery.

So Congress felt they were powerless to regulate it within established states. However, what laws governed a territory prior to statehood were set by Congress, so slavery within territories was entirely within their power. Also, it was entirely up to Congress how to decide to let in new states, so they were perfectly within their rights to not allow entry of a state to the union if they didn't like what that state's constitution said about slavery.

The hole in this argument is that a Constitutional Amendment can regulate behavior within states. So Slavery can be banned that way. However, that requires two thirds of either states or their Senators.

So as long as at least one third of states stayed slave as states were added, slavery couldn't be banned in them.

But again, a majority vote in Congress is used to regulate territories and admit states. So in the long run, simply having a minority of slave states sending Senators to Congress makes slavery vulnerable. This is why the slave states felt that every new free state added to the union had to be balanced with at least one more slave state.

  • I'm not sure that a constitutional amendment could have regulated behavior within states prior to the civil war. I suspect that the Southern states would have argued the point on the basis of the 10th amendment and the nullification crisis; they would have lost the argument for the reasons you list, but I think your presentation overstates the argument. (You could argue that a constitutional amendment can ban activity within the union, but prior to the civil war that is rather different from within states).
    – MCW
    Dec 24, 2015 at 2:19
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    All that said, solid argument to a complicated question. Lots of assumptions that need examination.
    – MCW
    Dec 24, 2015 at 2:19
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    @HenningMakholm - In theory, that might seem like a problem. However, in practice it was understood that slavery was nearly politically impossible to get rid of in a state once it was established there. Confiscating the property en masse of the richest (and thus most powerful) citizens in a state is not a trivial task. So the slave states (who were the ones most politically exercised about keeping the balance) didn't have much of a worry about defectors.
    – T.E.D.
    Dec 24, 2015 at 14:18
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    @MarkC.Wallace - It really wouldn't have mattered, since nobody, North or South, was advocating any laws removing slavery in states that had it. Even Abolitionists were trying to change minds rather than laws. So a "wait and see" South would likely have kept slavery for quite some time. The resort to violence led to the acceptance of most in the North to finish the job via amendment before the war even ended.
    – Oldcat
    Jan 5, 2016 at 0:17
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    @T.E.D - actually, this isn't the case. The New England states and mid Atlantic states all eliminated slavery without huge problems by about 1820. The usual gambit was to ban it in 10-20 years, giving holdouts ample time to move or to sell their slaves south beforehand. By the Civil War, Delaware was nearly bare of slaves, and Maryland was losing ground in most areas of the state.
    – Oldcat
    Jan 5, 2016 at 0:20

Free and slave states were determined largely by climate. That is, land south of the Ohio River and the Mason-Dixon Line (between Pennsylvania and Maryland) was suitable for cash crops such as tobacco, cotton and sugar,and therefore slavery, while land north of it was more suitable for subsistence farming (free labor).

The other factor is that with slavery being so contentious, legislators tried to keep a reasonable balance of states. The 13 Colonies had seven free and six slave states (divided by the Mason-Dixon Line). Vermont was the eighth free state, and Kentucky and Tennessee the seventh and eighth slave state. As long as the "U.S." was just east of the Mississippi River, one could expect that Ohio, Indiana and Illinois would be free, while the future Missisippi, Alabama, and Florida would be slave.

The Louisiana Purchase changed all that. Because the territory is "funnel shaped" out of 12 future states, only Louisiana, Arkansas, and Missouri were likely slave states, the remainder probably free (with the possible exception of Kansas). The acquisition of Texas changed things back; Southerners thought that it could be broken up into "several" slave states to balance the excess Louisiana Territory free states.

In determining the status of "free" or slave states, the U.S. Congress by and large followed the will of the people in the state, although there were some attempts to "stuff" states (e.g. Kansas) with pro slavery people.

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    Not so much cotton or sugar in the northern tier of the South, and slavery really got its start in North America with labor-intensive tobacco farming in Virginia and Maryland; tobacco would have been the cash crop of choice in the border states up to the Civil War and even after it.
    – Spencer
    May 30, 2017 at 2:41
  • And tobacco grows quite nicely as far north as South-Western Ontario, being a big cash crop in the London area right up to the present day. May 30, 2017 at 10:32

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