For a quick summary version of the Missouri Compromise, we'll turn again to he 1960 edition of The Beards' New Basic History of the United States by Charles, Mary and William Beard:
Under its power to make "all needful rules and regulations" respecting territory belonging to the United States, could Congress lawfully exclude slavery from such territory? That subject was not mentioned in the document, but it was squarely raised in 1818-20 during a dispute over the admission of Missouri to the Union as a slave state. By that time Northern states bad abolished slavery within their borders, or had provided for gradual abolition, and anti-slavery sentiment was growing insistent in the country.Jefferson was no longer holding public office in 1820. But he continued to follow public events and communicate with political leaders. And he made a statement that became perhaps the most famous comment on the Missouri Compromise, as we see in this excerpt from the Britannica Online article on it (internal links omitted):
Opposition to the admission of another slave state was outspoken in Congress and a deadlock occurred on the issue. After many angry words had been uttered, the standstill was broken by a compromise: Missouri was to be admitted with slavery, and the balance of political power maintained by the admission of Maine as a free state. In addition, as a part of the compromise, slavery was to be prohibited in the rest of the Louisiana Territory north of the line 36° 30'.
On the one side it was claimed that Congress bad no authority to prohibit slavery in this territory. On the other, attention was called to the fact that the Northwest Ordinance, adopted in 1787 and ratified by Congress in 1789, had excluded slavery from the Northwest Territory; so it was argued that Congress could lawfully do this under the Constitution.
The territory of Missouri first applied for statehood in 1817, and by early 1819 Congress was considering enabling legislation that would authorize Missouri to frame a state constitution. When Rep. James Tallmadge of New York attempted to add an antislavery amendment to that legislation on February 13, 1819, however, there ensued an ugly and rancorous debate over slavery and the government’s right to restrict slavery. ...In 1820, the Union contained 22 states, equally divided between slave and free states. Missouri territory was part of the enormous Louisiana Purchase that Jefferson made from France in 1803. The Missouri Compromise, brokered by House Speaker Henry Clay (1757-1852). As noted above, the agreement involved:
Although slavery had been a divisive issue in the United States for decades, never before had sectional antagonism been so overt and threatening as it was in the Missouri crisis. Thomas Jefferson described the fear it evoked as “like a firebell in the night.” Although the compromise measures appeared to settle the slavery-extension issue, John Quincy Adams noted in his diary, “Take it for granted that the present is a mere preamble—a title page to a great, tragic volume.” Sectional conflict would grow to the point of civil war after the Missouri Compromise was repealed by the Kansas-Nebraska Act (1854) and was declared unconstitutional in the Dred Scott decision of 1857. [my emphasis]
- Admission of Missouri to the Union as a slave state
- Admission of Maine to the Union as a free state, preserving the 50-50 balance among slave and free states and therefore the balance of representation between the two in the US Senate.
- Exclusion of slavery from all parts of the Lousiana Territory north of the parallel 36° 30'
It was the later that particularly disturbed Jefferson, as Jonathan Earle explains by Jonathn Earle in "The Political Origins of the Civil War" OAH Magazine of History 25:2 (2011):
The debates and threats of disunion went to the core issue of the struggle for power in Congress between representatives of the northern and southern states and, related to that, the ability of Congress itself to decide the fate of the territories added to the United States. These were difficult issues to solve, and incidentally ones put off into the future by the founders. This is why the aging Thomas Jefferson confided to a correspondent his fears that the line created by the Compromise would endanger the future of the nation: "this momentous question, like a fire bell in the night, awakened me and filled me with terror. I considered it at once as the knell of the Union ... a geographical line, coinciding with a marked principle, moral and political, once conceived and held up to the angry passions of men, will never be obliterated."There is nothing in that comment of Jefferson's that countenances the legitimacy of secession, for slavery or any other matter. Jefferson's seriousness about his consistent opposition to slavery has been brought into question, largely because his treatment of his own slaves. But he did oppose the institution of slavery as a matter of policy. And if his position on the Missouri Compromise represented any retreat from his antislavery policy position, he managed to keep it well concealed.
Dumas Malone devotes a chapter of the sixth and final volume of his Jefferson biography, The Sage of Monticello (1981) to Jefferson's reaction to the Missouri Compromise. He writes that the Burr conspiracy during Jefferson's Presidential Administration heavily influenced the former President's view of the context of the threat of disunion. In particular, Jefferson worried that the most likely scenario in which the Union would be split would be by secession of western states. Which is what Burr had tried to pull off.
A second important consideration is Jefferson's hostile view of the threat of states seceding was heavily influenced by his experience with the New England Federalist threats to secede in support of Britain in the War of 1812, a threat that was represented by the Hartford Convention of . A very much related consideration for him was that the Federalists still around in 1820 were supporting the effort to ban slavery in Missouri, which touched off the crisis resolved by the Missouri Compromise. In 1821, he wrote (letter to Henry Dearborn 08/17/1821):
I rejoice with you that at length a member of our Union. Whether the question it excited is dead, or only sleepeth, I do not know. I see only that it has given resurrection to the Hartford convention men. They have had the address, by playing on the honest feelings of our former friends, to seduce them from their kindred spirits, and to borrow regaining power under political distinctions, they have adroitly wriggled into its seat under the auspices of morality, and are again in the ascendency from which their sins had hurled them.Jefferson is arguing that that Federalists and their political successors had used the Missouri crisis to recuperate their reputation and take votes from the Jefferson-Madison-Monroe Republicans by using the antislavery issue ("under the auspices of morality"). He clear assumption, emphasized by Malone, was that the concern over slavery expressed by the opponents of the Jeffersonians was political cynicism.
But a third reason was Jefferson shared with most pre-Civil War Abolitionists an unrealistic idea of what emancipation of the slaves would look like. More specifically, Jefferson - and Lincoln prior to the war - supported the program of sending freed slaves to Africa. This approach was never especially attractive to black Abolitionists like Frederick Douglass. Jefferson's assumption, as he expressed in letters around this time, was that if the free states forced emancipation onto the slave states, that it would result in something like a civil war of blacks against whites, with the latter being in deadly danger in that situation.
In this latter sense, Jefferson was taking a proslavery position, in the immediately practical context. He was likely assuming that the emancipation he hoped to see in Virginia and other Southern states would take place in a similar way to what had occurred in the free states of 1820: emancipation enacted by the states in a process which included a relative reduction in the number of black people present. But he also understood that the process he envisioned would require a mass emigration of blacks, which in reality would have had to have been a mass expulsion.
In the same letter in which his famous fireball in the night metaphor appeared, he wrote this (letter to John Holmes, 04/22/1820), "Of one thing I am certain, that as the passage of slaves from one State to another, would not make a slave of a single human being who
would not be so without it, so their diffusion over a greater surface would make them individually happier, and proportionally facilitate the accomplishment of their emancipation, by dividing the burthen [sic] on a greater number of coadjutors." Such a process of "diffusion" was what had occurred in the states which by 1820 had abolished slavery at the state level. It's worth noting here that the issue in the infamous Dred Scott decision in 1857 was whether taking a slave into a free state abolished the slave's legal state of servitude.
In that same letter, he used dignified language to express the terror whites in the slaveowning states feared from slave revolts, as well as from the abolition of slavery in a state in which "dispersion" or some other kind of mass exit of black people:
... there is not a man on earth who would sacrifice more than I would to relieve us from this heaveymore than I would to relieve us from this heavy reproach [the insitution of slavery], in any practicable way. The cession of that kind of property, for so it is misnamed, is a bagatelle which would not cost me a second thought, if, in that way, a general emancipation and expatriation could be effected; and gradually, and with due sacrifices, I think it might be. But as it is, we have the wolf by the ears, and we can neither hold him, nor safely let him go. Justice is in one scale, and self-preservation [i.e., of the white population in the slave states] in the other. [my emphasis]