Final answer:
Sectionalism caused rising tensions between the North and South due to disagreements on the expansion of slavery into new territories, economic concerns, and the fear of political domination by the opposing side, ultimately contributing to the unraveling of national unity and the onset of the Civil War.
Step-by-step explanation:
Sectionalism was a major cause of rising tensions between the North and South leading up to the Civil War. As the United States expanded westward, intense debates sparked over whether new territories would permit slavery. The North, increasingly influenced by antislavery sentiments, opposed its expansion, arguing that it suppressed wages and hindered white Americans from achieving economic independence. The South, on the other hand, relied heavily on slavery for its agricultural economy and feared that without the ability to expand the institution, they would be overtaken politically by abolitionists and be vulnerable to slave insurrections.
Several sectional crises, including the nullification crisis and the ideological debates over protective tariffs, further exacerbated these divisions. Northern accusations of a 'slave power conspiracy' and southern fears of an 'abolitionist agenda' polarized the nation. By the late 1850s, figures like William H. Seward were addressing these as an 'irrepressible conflict,' foreshadowing the inevitability of nationwide decision-making regarding slavery. This deep ideological divide, with neither side willing to compromise, led to the gradual unraveling of national unity and set the stage for the Civil War.