In 1818, Illinois was admitted as a free state and in 1819, Alabama came in as a slave state. This increase of the number of slave states did not increase the number of slaves, as the slaves introduced into them came from the older slave states. If any slaves were introduced from Africa or any foreign country, it was by such evasion of the laws as will take place under any government, and they were not so introduced to any appreciable extent.

In 1819, towards the close of the 15th Congress, a bill was introduced into the House of Representatives to authorize the erection of the state of Missouri out of part of the territory of that name, and on motion of Tallmadge, of New York, a clause was inserted in the bill prohibiting the further introduction of slaves and granting freedom to the afterborn children of those already there, on arriving at the age of twenty-five, the proposition being carried by a vote of 87 to 76. This proposed restriction caused a very excited debate, in the course of which Cobb, of Georgia, said that "a fire had been kindled which all the water of the ocean could not put out, and which only seas of blood could extinguish;" he did not "hesitate to declare that if the northern members persisted, the Union would be dissolved." The bill, however, passed the House with the restriction, but in the Senate, the latter was stricken out, the clause prohibiting the further introduction of slaves by a vote of 24 to 16, and the one freeing the children by a larger vote, there being only 7 votes for retaining it. The House refused to concur with the Senate and the bill was lost.

At the same time the Missouri bill was introduced, another bill was presented for establishing Arkansas territory out of that part of the Missouri territory south of 36° 30´, and a clause was inserted into it granting freedom to all afterborn children of slaves, at the age of twenty-five, but a clause prohibiting the further introduction of slaves was defeated by a vote of 70 to 71 and the clause for freeing the children of those already in the territory was stricken out. Taylor of New York then proposed to add a proviso to the bill that neither slavery nor involuntary servitude should exist in any of the territories of the United States north of 36° 30´, but his motion was defeated and the bill for organizing Arkansas Territory passed both houses without any restriction.

Before the meeting of the next Congress, Massachusetts authorized the formation of the District of Maine into a state and a Constitution was adopted by the people in that district for that purpose. In the meantime there was much agitation in the North upon the subject of excluding slavery from the territory west of the Mississippi. Upon the meeting of the 16th Congress, a bill was introduced to authorize the people of Missouri to frame a State Constitution, but on motion of Taylor, the author of the proposed proviso excluding slavery from the territories north of 36° 30´, a committee was appointed to consider the subject of prohibiting slavery west of the Mississippi, and the Missouri bill was postponed to await the action of the committee.

A bill had been introduced for the admission of Maine—and after the defeat of a motion to postpone it until the Missouri bill came up—was passed. When this bill came up in the Senate, a clause for the admission of Missouri, was attached to it, after the defeat of a motion to insert in the latter a proviso for the prohibition of slavery, and Thomas, a senator from Illinois, then proposed an amendment prohibiting the introduction of slavery into any of the remaining territory north of 36° 30´, which was adopted by a vote of 34 to 10; the senators from Virginia, South Carolina, Georgia, Indiana, and one senator from North Carolina and Mississippi each voting in the negative. The bill was then passed by a vote of 24 to 20, all the senators from the slave states and the two from Illinois voting in the affirmative, and those voting in the negative being from the free states.

The House refused to concur in the Senate's amendment, and the Senate adhered, therefore a committee of conference was appointed. In the meantime, the House had been debating the Missouri bill, and pending the conference it was passed by a vote of 93 to 84 with a clause prohibiting the further introduction of slaves. When this bill went to the Senate, the prohibition was stricken out and the Thomas proviso attached, and it was then passed and returned to the House. The Committee of Conference at the same time reported recommending that the Senate recede from its amendment to the Maine bill and that the House pass the Missouri bill as amended by the Senate. The House agreed to the amendment to the Missouri bill, striking out the clause for prohibiting slavery, by a vote of 90 to 87, and to that inserting the Thomas proviso, by a vote of 134 to 42, 35 of the latter being Southern members who objected to the proviso as unconstitutional, and 5 being Northern men who objected because it did not go far enough. The Senate receded from its amendment to the Maine bill and both bills were thus passed.

President Monroe signed the Missouri bill after much hesitation, upon having his scruples as to the constitutionality of the proviso removed, and upon being assured that the restriction as to the territories extended to them only while in the territorial condition.

The bill in relation to Maine admitted that state into the Union at once, but that in regard to Missouri was a mere act enabling the people to frame a Constitution, and a joint resolution for the admission of the state after the formation of the Constitution was still necessary.

When the Constitution was presented at the next session of Congress, it was found to contain a clause requiring the legislature to pass laws to prevent free persons of color from settling in the state, and as the admission of Maine was complete, the Northern members took occasion to object to the admission of Missouri because of this clause, though Ohio and Indiana had passed laws forbidding the settling of free persons of color in those states, and there was an old law of Massachusetts to the same effect, still unrepealed. A resolution offered in the House for the admission of Missouri, with its Constitution as it stood, was defeated by a vote of 78 to 93, those voting in the negative being Northern members. After much discussion and excitement and the defeat in the House of an effort to compromise the question, on motion of Mr. Clay, a joint committee was appointed to take the subject into consideration, and this committee reported a joint resolution for the admission of Missouri, after the state legislature should have given a solemn pledge, that the Constitution should not be construed to authorize any act and that no act should be passed "by which any of the citizens of either of the states should be excluded from the enjoyment of any of the privileges and immunities to which they are entitled under the Constitution of the United States." The President being authorized to announce by proclamation, the adoption of the pledge, and Missouri then to become a state in the Union, this resolution was adopted, the vote being 86 to 52 in the House, all the votes in the negative, excepting four, being given by Northern members and the four Southern members not being willing to submit to the concession. Since the rejection of the proposition for compromise in the House on the same basis, news had been received of the final ratification by Spain of the treaty for the cession of Florida, and as by that treaty the United States relinquished all claim to Texas, thus reducing the whole of the territory south of 36° 30´ and west of the Mississippi to the Territory of Arkansas, comprising the present state of Arkansas and the small tract of Indian country west of it, while there remained an immense domain north of that parallel, stretching across the Rocky Mountains to the Pacific, a few Northern members were induced to cast their votes for the last proposition, thus securing its passage.

The required pledge was given by the legislature of Missouri, and that state was thus admitted into the Union in 1821. For a long time, the arrangement by which the passage of the enabling act for Missouri was secured, was called compromise, and the line of 36° 30´ was called "The Missouri Compromise Line." The subject was fully explained by Mr. Clay in the Senate in 1850, and it will be seen that the arrangement was no compromise at all, but was merely one of those legislative expedients often adopted to secure the passage of a measure. As it passed, the restriction was merely a legislative enactment, liable to repeal at any time like any other law. But few of the Northern members agreed to the arrangement, and at the very next session of Congress, the great mass of them repudiated the idea of its being a compromise by voting against the admission of Missouri, upon a mere pretext.