"In the long run, the most significant division of American opinion exacerbated by the war in Mexico was that between the North and South. The doctrine of America's manifest destiny had not sprung originally from a slave power conspiracy but from policies with nationwide appeal and deep cultural roots. When James Knox Polk came into office, territorial expansion did not constitute a sectional issue but a party one. . . . Polk did not share Calhoun's disposition to view all matters in terms of their impact on the slavery question. Nevertheless, as his term went by, his administration increasingly appeared narrowly southern in outlook. The president's imperialist objectives came to prompt a bitter sectional dispute over slavery's extension, bearing out Calhoun's foreboding."According to the passage, which of the following best explains the most important effect that the Mexican-American War had on the Un

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