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U.S. war with Mexico opened vast new western lands whose future, free or slave, immediately intensified sectional conflict. Debates over the status of the Mexican Cession made slavery expansion the crisis of the era. (McPherson, 18–19)
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The Proviso objective was to prohibit slavery in territory taken from Mexico. Although it failed, it showed hardened sectional voting lines and placed slavery expansion at the center of national politics, worsening partisan tensions. (McPherson, 19)
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Clay’s compromise admitted California as free, strengthened the Fugitive Slave Act, and reorganized the West. It briefly calmed tensions but deepened divisions as each section felt the other had gained unfair advantages. (Varon, 199–200)
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Aggressive enforcement alarmed Northerners, who saw federal power used to protect slavery. Highly publicized arrests fueled resistance, radicalized moderates, and pushed antislavery sentiment into mainstream debate, also fed into Southern Power conspiracy's. (Varon, 235–236)
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Placed in this timeline as it shows early organized Southern nationalism. These conventions promoted pro-slavery trade and argued the South needed independent economic policies, laying a structure for the idea of secession as sectional distrust worsened (Varon, 273-275, 314)
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This was key to the Republican Party origins. This manifesto condemned the Kansas-Nebraska Act as a pro-slavery plot, gathering Northern outrage and helping to create a new political coalition dedicated to stopping slavery's expansion (McPherson, 122-123)
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Shows the Northern backlash to the Fugitive Slave Act. A slave placed on trail had the court house stormed by anti-slavery Northerner's allowing him to escape. federal troops forcibly returned Burns to slavery, shocking Boston. Many Northerner's saw this as proof slavery endangered their own liberties (Varon, 247)