Final answer:
The Missouri Compromise was a critical agreement in 1820 that admitted Missouri as a slave state and Maine as a free state, preserving the balance of power in the Senate and setting a boundary for the expansion of slavery.
Step-by-step explanation:
The Missouri Compromise, also known as the Compromise of 1820, addressed the issue of maintaining a balance of power between slave states and free states. It was orchestrated by Speaker of the House Henry Clay and admitted Missouri as a slave state and Maine as a free state, maintaining the balance of slave and free states in the Senate. Additionally, it included provisions like the Thomas Proviso, which banned slavery north of the southern boundary of Missouri at the 36°30' line within the Louisiana Purchase territory.