applied for statehood in 1817 it saw to disrupt the balance of power within the U.S. Senate
In the early 1800s the northern, or 'free' states, were more populous and industrial than the southern, or 'slave' states. Thus, the U.S. House of Representatives was run primarily by the north. The only thing stopping the north from passing anti-slavery legislation was the Senate. While the north had more people, it still had the same number of states as the south, and just as many senators.
Missouri was going to join the Union as a slave state, tacking on two more votes for southern interests. The north couldn't have that. Allowing a slave state in the western lands and in the area of the Louisiana Purchase was a bad precedent, also it would tip the scales in the Senate toward the south. Neither side wanted a state to be admitted to either side.
In late 1819 and early 1820 there came a solution to the dilemma. Maine, which was previously a part of Massachusetts, was admitted to the Union as a free state, and Missouri was admitted to the Union as a slave state. A pair of states added almost simultaneously worked for both sides of the Mason-Dixon Line. But what about the precedent of slavery in the western lands.
Henry Clay, a northern Senator who sold the idea in Congress, also sold the idea of creating a border between the future slave and future free states. Missouri's southern border, a line along the 36th parallel, was accepted as the border. Future territories above that line were to be free, and below were to be slave.
This Compromise of 1820 was in effect until the 1850 when a new compromise was written and the Dred Scott case was decided.