Missouri Compromise of 1820: Terms, Summary & Definition

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The Missouri Compromise passed Congress in 1820. It admitted Missouri to the Union as a slave state and barred slavery from the Louisiana Territory north of the 3630' parallel.

We also recommend watching Henry Clay and the Missouri Compromise of 1820 and The Constitutional Convention: The Great Compromise


When new states were added to the Union in the early years of United States, there was concern over whether they would be admitted with or without slavery. The Missouri Compromise was one of the most important legislative actions of the 19th Century in Congress because of the precedent it set regarding such matters.

Louisiana Territory

In 1803, President Thomas Jefferson secured one of the great successes of his presidency. That year, the Louisiana Territory was acquired from France with the Louisiana Purchase. This land stretched from New Orleans all the way north to modern day Minnesota, northwest to modern day Montana, and southwest to modern day Arizona. It massively increased the size of the United States. With this increase in size, however, came major problems.

During the first several decades of the 19th Century, there was a delicate balance between southern slave states and northern free states in Congress. Because each state received two senators, this meant that slave states and free states were equally represented in Congress. However, with the additions of new states, questions arose regarding how that balance would be affected.

Map of U.S. Territories in the 19th Century

Missouri Compromise

When Missouri applied for statehood in 1819, James Tallmadge, a representative from New York, put forward an amendment that would eradicate slavery in Missouri over time, meaning that Missouri would be joining the Union as a free state. The amendment, known as the Tallmadge Amendment, passed the U.S. House of Representatives along with a Missouri statehood bill, but it died in the U.S. Senate because of a lack of support.

A second attempt was made with two separate measures from the U.S. House in 1819 that admitted Missouri as a slave state and admitted Maine as a free state (Maine had previously been a part of Massachusetts). The U.S. Senate essentially combined these two measures into one bill and passed the act in 1820. Before it was passed, however, an amendment was added that made the 36°30' parallel an important dividing line in the Louisiana Territory. This parallel was the southern boundary of Missouri. According to the legislation, this line extending westward into the new territory would determine whether slavery would or would not be tolerated in new lands. Everything north of the line would be free soil, and everything south would be open to the spread of slavery. This became a part of the final version of the Missouri Compromise of 1820.

Legacy of the Missouri Compromise

For over thirty years, the Missouri Compromise governed how slavery was dealt with in new lands and new states admitted to the Union. It was an important dividing line that helped to keep the nation together during sectional tensions over slavery. Yet, by the 1850s, the situation had become so tense that the compromise line could no longer hold things together.

In 1854, Senator Stephen Douglas from Illinois introduced a measure that would lead to statehood for Kansas and Nebraska. However, rather than following the long held precedent of them being free states (they are north of the 36°30' parallel), Douglas's bill stated that it would be up to the people of the states to determine whether slavery would or would not be allowed. This bill passed, essentially overturning the Missouri Compromise line. It is known as the Kansas Nebraska Act.

In 1857, the Missouri Compromise was further limited, but this time it was by the Supreme Court. In the Dred Scott decision, Chief Justice Roger Taney ruled that the Missouri Compromise was unconstitutional because it prevented slave owners from exercising their property rights. Many opponents of slavery quickly realized that this decision was a means of expanding slavery.

Together, the Kansas Nebraska Act and the Dred Scott decision set off the political movement that became the Republican Party. The Republicans' sole uniting belief was that the Kansas Nebraska Act was wrong, the spread of slavery should be stopped, and in many cases, the Missouri Compromise needed to be re-instituted.

Thus, the Missouri Compromise loomed large over the sectional tensions in the years before the American Civil War. For over thirty years it helped to keep the country together, and its dissolution in the 1850s was a major turning point on the road toward secession and Civil War, all because of the issue of slavery in the United States.

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