The Missouri Compromise was an attempt to head off the slavery debate which was gradually heating up. The issue was not so much the existence of slavery, as its extension into the West. Stephen A. Douglas proposed the Compromise as part of a plan to have the Eastern Terminus of the Transcontinental Railroad in Chicago in his home state.
Missouri petitioned to come into the Union as a slave holding state. This would have upset the balance of slave and free states in the Union. The free states already had a majority in the House of Representatives; but the Senate was evenly divided. Were the slave states to achieve a majority; they might institutionalize slavery with a constitutional amendment. If the free states were to gain a majority, they might attempt to eliminate slavery altogether, again by Constitutional Amendment (both sides agreed that as onerous as slavery was, it was constitutionally sanctioned. The plan called for the admission of Missouri as a slave state, and Maine as a free state; whereby the division in the Senate would be preserved. It also provided that slavery would not exist above the parallel which constituted the bottom of the Missouri state line.
The compromise is important in that it delayed the slavery debate for a short time. It did not end it. Later, the compromise was declared unconstitutional by Justice Roger Taney in the Dred Scott decision, and the slavery debate heated up anew.
The Missouri Compromise was significant because it helped to reduce tensions between the North and the South. By doing so, it helped to delay the Civil War. It is also significant, however, because it is the first real manifestation of those tensions.
The North and the South had been evenly split in terms of number of states until 1820. At that point, Missouri wanted to enter the Union as a slave state. That would have given the slave states more power in the Senate than the free states had. This worried the free states and was the first real sign of tension between the two sections.
The Missouri Compromise solved the problem by letting Maine in as a free state to maintain the equality in numbers of the free and slave states. It also set out which parts of the Louisiana Purchase would become slave territory and which parts would be free. By doing so, it reduced the tensions between the North and the South.
The Missouri Compromise included that Maine would enter the Union as a free state. Missouri would enter the Union as a free state. The remaining territory of the Louisiana Purchase would be closed to slavery.