Why did the Mexican Cession renew tensions about slavery between northern and southern states?

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rrteacher | College Teacher | (Level 2) Educator Emeritus

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By the middle of the nineteenth century, the most contentious issue in American politics was whether slavery should be allowed to spread into federal territories, and ultimately into new states. Southern states, outnumbered in terms of population, were thus outvoted in the House of Representatives. But more importantly, they worried about the potentially damaging effects of non-slave states on their borders. The annexation of Texas, and the outbreak of the Mexican War that followed, caused these issues to come to the forefront with an intensity not seen since 1820. Even before the war's beginnings, a northern Whig representative, David Wilmot, attempted to add a proviso to a military spending bill that would ban slavery from any territories gained as a result of the war. A heated debate ensued that continued even after the war that culminated when California, part of the massive tract of land known as the Mexican Cession, applied for admission to the Union as a free state. Southern states demanded protection for slavery in the territories and new states, and Northern Free Soilers, a faction of the Whigs that had been opposed to war in the first place, bitterly disagreed. The Compromise of 1850 that ended the controversy also contributed to future strife by introducing the concept of popular sovereignty for determining the future of slavery in the territories and by passing into law a new and stricter fugitive slave act.