The English arrived on the coast of what is now Virginia in 1607 and established the first permanent English colony in America. They named their settlement Jamestown in honor of the reigning monarch, King James I (who was also King James VI of Scotland).
In 1619, the English brought the first documented African slaves to Jamestown. At the time, England was at war with Portugal and the Africans, originally from Ndonga, a kingdom located in present day Angola, were captured from the Portuguese and transported to the American colony. They were used to help establish and expand the agriculture of Jamestown, especially the growing of tobacco.
From that time on, slaves played a key role in the colonization and growth of America. (Just a side note, by the time Abraham Lincoln signed the Emancipation Proclamation, Africans weren’t the only slaves in America. Many Irish immigrants also ended up as slaves, but you rarely hear about them.)
Over the next century and half, America grew in size and the population rapidly increased. The extensive use of slaves helped that growth, mainly in the south. By the time America won its independence from Great Britain in 1783, there was already a growing division between the northern and southern states over the issue of slavery.
Of the original 13 states, 7 were free and 6 were slave. Georgia, South Carolina, North Carolina, Virginia, Maryland and Delaware relied heavily on slavery to sustain their economy. New Jersey, Pennsylvania, New York, Connecticut, Rhode Island, Massachusetts and New Hampshire for the most part banned slavery.
As new colonies and territories became states, the issue of them being a slave or free state became an issue of power in the Senate. In 1791, New Hampshire was divided, allowing Vermont to become the 14th state, which now further tipped the balance with 8 free states to only 6 slave states.
A year later in 1792, Kentucky became the 15th state in the nation and they were a slave state. Four years later to the day, June 1, 1796, Tennessee was admitted as a state and the balance of free versus slave states was equal at 8 each.
The balance between free and slave states continued to be an issue as even more states were admitted to the nation with neither side wanting to relinquish numbers to the other.
In 1803, Ohio became the 17th states and once again tipped the balance in favor of the free states. The balance in the Senate remained tipped in favor of the free states for 9 years until Louisiana was admitted as a slave state in 1812.
In 1816, the balance was once again tipped in favor of the northern free states with the admission of Indiana. A year later in 1817, the balance was evened out once again with the admission of Mississippi as a slave state, bringing the total number of states to 20.
By now the issue of slave versus free was becoming a very hot and contentious subject in the Senate. With each new admission to the nation, tempers flared, voices raised and bitter arguments were not unheard of. Every time the northern members of the Senate would push for the admission of a new free state, the southern members would quickly retaliate with the push for a new slave state.
In December 1818, southern tempers flared when Illinois was admitted as a free state, again tipping the balance of power. The issue of free and slave states came to a head in 1819 when southern members of the Senate pushed to get Missouri admitted as a slave and northern members of the Senate pushed to make Missouri a free state.
By the end of 1819, somehow Alabama had been admitted as a slave state, again equaling the power in the Senate. However, this made the admission of Missouri even more of a hotly contested issue. If Missouri was granted admission to the nation as a slave state, it would have been the first time that the slave states would have the majority in the Senate.
The argument of over Missouri blossomed into one that involved the authority of Congress versus the authority of the states. Sen. Rufous King (NY) argued that congress indeed had the authority to rule on the legality of slavery. Sen. William Pinkney (MD) argued that the states retained that power as they had when the nation first formed.
The battle continued from December 1819 until March 1820 when a compromise was reached. It involved allowing Massachusetts to allow its northern territory to be admitted as the free state of Maine and Missouri would be admitted as a slave state, thus keeping the delicate balance of power even in the Senate.
The compromise also established the 36º30’ parallel to be a future dividing line between free and slave states. Any territories south of the parallel would be admitted as slave states and those north would be admitted as free states.
On this day, March 3, 1820, the Missouri Compromise was passed by Congress. Twelve days later, Maine was admitted as the 23rd state. Missouri was admitted next in August 1821 and the balance of power in the Senate was equal once more.
The Missouri Compromise remained in effect until it was challenged in 1854 with the admissions of Kansas and Nebraska. Even though they were both north of the parallel, they wanted to determine for themselves whether to be free or slave. Congress ended up repealing the Missouri Compromise with the enactment of the Kansas-Nebraska Act of 1854.
The repeal was fortuitous as the US Supreme Court ruled the Missouri Compromise as being unconstitutional as part of the famous Dred Scott Decision. In the decision, they stated that Congress had violated the Fifth Amendment rights of individuals by depriving them private property without due process of law.
Sources for the above include: Missouri Compromise; The Missouri Compromise; Missouri Compromise; Missouri Compromise; Congress passes the Missouri Compromise; MISSOURI COMPROMISE; SLAVERY IN AMERICA; A History of Jamestown; States by Order of Entry into Union.