In 1812, the "Lower Louisiana" was admitted to the Union as a state, "Upper Louisiana" was organized into the Missouri Territory. By 1818, Missouri had reached the point that its citizens petitioned for admission themselves. Since the institution of slavery had been an established element in Missouri since French colonial times, this would have created a situation where the U.S. Senate would have a majority of Southern slaveholding states. That prospect was extremely unwelcome in the North.
In addition to the economic question of slavery, there was a moral issue. Many in the north felt that the federal government should not allow slavery in its new territories and should move along a path to eventually destroy it. The House of Representatives put forward an amendment to the admission of Missouri that would prohibit the introduction of slaves into Missouri and freeing the children of slaves at the age of 25. Passed by the House in February, 1819, the Tallmadge Amendment was defended in the Senate by Rufus King of New York, who argued that the Constitution allowed the federal government to determine the conditions under which states would be admitted, and the Northwest Ordinance of 1787 provided the precedent for restricting or forbidding slavery.
Southern opposition in the Senate, however, doomed the Tallmadge Amendment to defeat. The Senate passed the bill admitting Missouri without the amendment, but it was rejected by the House, pushing the controversy into 1820.
Later in the year, Maine held a convention and requested admission as a "free" state. The Great Compromiser, Henry Clay, proposed the following elements of a sectional compromise:
Sectional balance was maintained - with the admission of Missouri and Maine there were 12 free states and 12 slave states. This established a precedent that would be followed for the next 30 years. New states would be admitted in tandemone slave, one free. The Missouri Compromise was canceled in 1854 with the passage of the Kansas-Nebraska Act .
That Missouri be admitted to the Union as a slave state (as the population of the territory apparently desired).
That slavery was to be prohibited from the new American territories in the Louisiana Purchase north of 36?30 north latitude (the southern boundary of Missouri). States to the south of the line (the new Arkansas Territory) would decide the slavery issue for themselves.
That Maine (formerly part of Massachusetts) be admitted to the Union as a free state.
---- Selected Quotes ----
Quotes regarding Missouri Compromise.
By John Quincy Adams
Take it for granted that the present is a mere preamble -- a title page to a great, tragic volume.
Noted in his diary, on passage of the Missouri Enabling Act in 1820