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The Missouri Compromise of 1820 was a congressional agreement that regulated the extension of slavery in the United States for thirty years. Under the agreement, the territory of Missouri was admitted as a slave state, the territory of Maine was admitted as a free state, and the boundaries of slavery were limited to the same latitude as the southern boundary of Missouri, 36°30′ north latitude.
By 1818 the rapid growth in population in the North had left the Southern states, for the first time, with less than 45 percent of the seats in the U.S. House of Representatives. The U.S. Senate was evenly balanced between eleven slave and eleven free states. Therefore, Missouri's 1818 application for statehood, if approved, would give the slave states a majority in the Senate and reduce the Northern majority in the House.
In 1819 the free territory of Maine applied for statehood. Speaker of the House henry clay of Kentucky saw this event as an opportunity to maintain the balance of free and slave states. He made it clear to Northern representatives that Maine would not be admitted without an agreement to admit Missouri. Clay persuaded opponents of slavery to drop efforts to ban it in the territories. In return, the Southern states agreed to limit slavery to the territory below 36°30′ north latitude. Under this provision the unsettled portions of the louisiana purchase north and west of Missouri would be free from slavery. The only area remaining for further expansion of slavery would be the territory that would become Arkansas and Oklahoma. To preserve the sectional equality in the Senate, Missouri and Maine were to be admitted to the Union simultaneously. Clay managed to pass the compromise in the House by a three-vote margin.
In 1821 Missouri complicated matters, however, by inserting a provision into its state constitution that prohibited free blacks and mulattoes from entering the state. Northern representatives objected to this language and refused to give final approval for statehood until it was removed. Clay then negotiated a second compromise that removed the offensive language from the Missouri constitution and substituted a provision that prohibited Missouri from discriminating against citizens from other states. Left unsettled was the question of who was a citizen. With this change Missouri and Maine were admitted to the Union.
Source: Statutes at Large, vol. 6 (1822), pp. 545–548, 645; Ben Perley Poore, ed., The Federal and State Constitutions, Colonial Charters, and Other Organic Laws of the United States, vol. 2 (1878), pp. 1107–1108.
An Act to Authorize the People of the Missouri Territory to Form a Constitution and State Government, and for the Admission of Such State into the Union on an Equal Footing with the Original States, and to Prohibit Slavery in Certain Territories
Be it enacted by the Senate and House of Representatives of the United States of America, in Congress assembled, that the inhabitants of that portion of the Missouri territory included within the boundaries hereinafter designated, be, and they are hereby, authorized to form for themselves a constitution and state government and to assume such name as they shall deem proper; and the said state, when formed, shall be admitted into the Union, upon an equal footing with the original states, in all respects whatsoever.
And be it further enacted, that the said state shall consist of all the territory included within the following boundaries, to wit: beginning in the middle of the Mississippi River, on the parallel of thirty-six degrees of north latitude; thence west, along that parallel of latitude, to the St. Francis River; thence up, and following the course of that river, in the middle of the main channel thereof, to the parallel of latitude of thirty-six degrees and thirty minutes; thence west, along the same, to a point where the said parallel is intersected by a meridian line passing through the middle of the mouth of the Kansas River, where the same empties into the Missouri River, thence, from the point aforesaid north, along the said meridian line, to the intersection of the parallel of latitude which passes through the rapids of the river Des Moines, making the said line to correspond with the Indian boundary line; thence east, from the point of intersection last aforesaid, along the said parallel of latitude, to the middle of the channel of the main fork of the said river Des Moines; thence down and along the middle of the main channel of the said river Des Moines, to the mouth of the same, where it empties into the Mississippi River; thence, due east, to the middle of the main channel of the Mississippi River; thence down, and following the course of the Mississippi River, in the middle of the main channel thereof, to the place of beginning: provided, the said state shall ratify the boundaries aforesaid. And provided also, that the said state shall have concurrent jurisdiction on the river Mississippi and every other river bordering on the said state, so far as the said rivers shall form a common boundary to the said state; and any other state or states, now or hereafter to be formed and bounded by the same, such rivers to be common to both; and that the river Mississippi and the navigable rivers and waters leading into the same shall be common highways, and forever free, as well to the inhabitants of the said state as to other citizens of the United States, without any tax, duty, impost, or toll, therefore, imposed by the said state
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