Final answer:
The North and South had major differences regarding slavery, primarily due to their distinct economic systems and political beliefs, which exacerbated the sectional crisis and contributed to the outbreak of the Civil War.
Step-by-step explanation:
The differences between the North and the South over the issue of slavery were numerous and significant, eventually leading to the American Civil War. In the 1850s, both Northerners and Southerners felt compelled to defend their position on slavery, arguing that the institution was central to their political and economic liberties, as they believed the Constitution and the legacy of the American Revolution guaranteed. The North did not depend economically on slavery, as their industrial economy differed from the South's agricultural one, which heavily relied on slave labor for crops such as cotton and tobacco.
Southerners feared the influence of the abolitionist movement and sought to protect their way of life, while Northerners saw a threat in what they believed to be a 'slave power conspiracy' looking to dominate national politics. By the end of the decade, the notion of irrepressible conflict became prominent, suggesting that the nation would have to choose to be entirely slave-owning or entirely free, indicating the deepening divide. Despite the extreme views, few on either side believed that these differences would lead to war.
The sectional crisis reached a peak after the U.S. victory over Mexico, as the expansion of slavery into new territories became a contentious issue. Northerners feared that the spread of slavery would favor Southern political dominance and harm the prospects of small, freeholding farmers in the West, while Southerners argued for their rights to take their property, including slaves, into new territories. This conflict over slavery expansion was one of the primary catalysts for the Civil War.