Final answer:
Southerners believed they had the legal right to bring slaves into new territories based on popular sovereignty and property rights, leading to stiff opposition from the North due to political, economic, and moral concerns. This divisive issue contributed to the Civil War.
Step-by-step explanation:
Why did southerners believe they should be allowed to bring slaves into lands north of the Missouri Compromise line? The correct answer is B) Legal reasons; with opposition. Southerners contended that popular sovereignty should apply to all territories, arguing for the right to bring their enslaved property wherever they chose. This belief was grounded in the idea that prohibiting the expansion of slavery ran counter to basic American property rights.
How did people in the North react to this? Northern reaction to the proposed expansion of slavery was generally one of opposition. They feared that the spread of slavery into new territories would lead to political dominance of the South, economic disadvantage for small farmers, and the spreading of an institution they believed to be immoral and contrary to the nation's values.
Within the broader context of U.S. history, the expansion of slavery was a divisive issue that eventually contributed to the outbreak of the Civil War. While some debates centered on the morality of slavery, much of the conflict arose from its economic and political implications, creating a deep rift between the North and the South.