Final answer:
The Missouri Compromise marked a departure from strict states' rights views by imposing federal restrictions on the expansion of slavery, notably with Maine's admission as a free state, Missouri's as a slave state, and the prohibition of slavery north of the 36°30' line.
Step-by-step explanation:
Departure from States' Rights with the Missouri Compromise
Agreeing with the Missouri Compromise represented a departure from the states’ rights position held by pro-slavery legislators because it established federal authority over slavery's expansion rather than leaving the decision entirely to the states. Before the Compromise, many pro-slavery advocates supported the notion that individual states possessed the right to determine the legality of slavery within their borders. However, the Missouri Compromise, brokered primarily by Henry Clay, introduced federal legislative action that directly impacted the spread of slavery, with Maine entering as a free state, Missouri as a slave state, and the establishment of the 36°30' line prohibiting slavery in the remainder of the Louisiana Purchase north of this latitude. This marked a significant shift because it was an agreed-upon limit imposed by the national government, which partially restrained the states' ability to make decisions about the institution of slavery.
The inclusion of Maine as a free state was meant to maintain the balance of power between free and slave states in the Senate, which was of critical importance to southern legislators who feared the loss of influence in Congress. The establishment of the 36°30' line was a clear demarcation that limited the territorial expansion of slavery, a matter which had been previously debated predominantly at a state level. Even though the Compromise was designed to prevent future conflicts and it provided explicit federal instructions about slavery in new territories, it effectively introduced a national framework which each new state entering the Union would have to consider, thus reducing the autonomy of states to determine their own slavery status