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Fighting began between United States and Mexican forces near the Rio Grande after the annexation of Texas. President James K. Polk described the conflict as defensive, though many Americans believed it was driven by expansionist goals. The war raised the urgent question of whether slavery would be allowed in newly acquired western lands, making territorial expansion the central political issue of the era. (McPherson, 45–48)
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The Wilmot Proviso proposed banning slavery in any territory acquired from Mexico. Although it never became law, the proposal revealed sharp sectional divisions in Congress, as representatives increasingly voted along regional lines. The debate showed that compromise over slavery’s expansion was becoming more difficult and that the issue dominated national politics. (McPherson, 56–58)
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The Treaty of Guadalupe Hidalgo ended the Mexican–American War and transferred vast territories from Mexico to the United States, including California and much of the Southwest. While the treaty completed American territorial expansion, it intensified sectional conflict by forcing Congress to debate whether slavery would extend into the new territories, deepening divisions between North and South. (Varon, 120–123)
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The Free Soil Party was created to oppose the expansion of slavery into western territories. It united former Democrats, Whigs, and antislavery activists who believed slavery threatened free labor and republican values. Although the party was short lived, it reshaped national debate by placing slavery expansion at the center of political life. (McPherson, 135–137)
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Southern leaders gathered at the Nashville Convention to discuss their response to federal policies on slavery. Although the convention did not endorse immediate secession, it showed that many Southerners believed disunion was a legitimate option. The meeting reflected growing Southern concern over political power and constitutional rights. (Varon, 187–191)
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The Compromise of 1850 admitted California as a free state while strengthening the Fugitive Slave Act and allowing popular sovereignty in western territories. The measures temporarily reduced sectional tension but failed to resolve the underlying conflict over slavery. Instead, the compromise delayed confrontation while increasing mistrust between North and South. (McPherson, 171–176)
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The Fugitive Slave Act required citizens and officials to assist in the capture of escaped enslaved people. The law sparked strong resistance in the North, where many viewed it as a violation of personal liberty. Its enforcement brought slavery directly into Northern communities and intensified sectional hostility. (McPherson, 178–182)
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The Southern Rights Convention brought together delegates from several Southern states to protest the Compromise of 1850 and federal interference with slavery. Although the convention did not lead to immediate action, it reflected growing Southern frustration and increasing willingness to consider resistance to federal authority. The meeting showed how sectional identity and disunionist rhetoric were gaining strength. (Varon, 206–210)
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The Gadsden Purchase transferred a strip of land from Mexico to the United States to support construction of a southern railroad route. Though limited in size, the purchase reflected ongoing sectional rivalry and showed that slavery related interests continued to influence decisions about territorial expansion. (McPherson, 216–218)
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The Know Nothing Party rose to national prominence by appealing to nativist fears of immigration and Catholic influence. Its rapid growth reflected widespread political instability during the 1850s as traditional parties weakened under the strain of sectional conflict. Although not centered on slavery, the movement revealed how national unity was unraveling in the years before the Civil War. (McPherson, 222–225)
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The Kansas–Nebraska Act allowed settlers to decide the issue of slavery through popular sovereignty and repealed the Missouri Compromise line. Rather than easing tensions, the law led to violence in Kansas and further divided the nation. It also weakened established political parties and increased sectional polarization. (McPherson, 229–233)
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The Ostend Manifesto was a secret diplomatic document arguing that the United States should acquire Cuba, even by force if necessary. Many Northerners viewed the proposal as evidence of aggressive pro slavery expansion. Its public release increased fears that slaveholding interests dominated national policy. (Varon, 248–251)
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The Supreme Court ruled in Dred Scott v. Sandford that African Americans were not citizens and that Congress could not ban slavery in the territories. Chief Justice Roger Taney argued that enslaved people were considered “beings of an inferior order” at the nation’s founding. The decision undermined compromise and intensified sectional conflict. (McPherson, 246–250)
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The Lecompton Constitution was a pro slavery document written for Kansas under unfair voting conditions. Although supported by President James Buchanan, most Kansas settlers rejected it. The controversy revealed the failure of popular sovereignty and further divided the Democratic Party, weakening national unity. (McPherson, 252–255)