Final answer:
The Missouri Compromise, occurring in 1820 and orchestrated by Henry Clay, admitted Missouri as a slave state, Maine as a free state, and established the 36° 30' parallel as the dividing line for slavery in the Louisiana Purchase lands.
Step-by-step explanation:
Missouri Compromise Overview
The historical event that included Missouri entering the Union as a slave state, Maine as a free state, and the establishment of the 36° 30' parallel as the dividing line between free and slave territories in the Louisiana Purchase lands is known as the Missouri Compromise. This compromise, which occurred in 1820, was largely the work of Henry Clay and was aimed at keeping the balance of power between free and slave states in the Senate. It also intended to soothe northern concerns about the expansion of slavery and to avoid disunion.
The Missouri Compromise was significant because it maintained the delicate balance of slave and free states within the United States Senate and was a key event in the tumultuous years leading up to the Civil War. This agreement was a pivotal moment in the history of the nation's struggle with slavery and sectional balance.