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

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The Mexican Cession gave the United States a great deal of land in what is now the southwestern part of the United States. The United States got land in the present-day states of California, Nevada, Wyoming, Utah, Colorado, New Mexico, and Arizona as a result of winning the Mexican-American War. The United States paid Mexico 15 million dollars for this land. However, acquiring this land renewed tensions between the free states and the slave states.

Tensions increased because both sides saw this land as an opportunity to gain more states that would either be free or have slaves. California quickly applied for statehood, mainly due to the large increase in population in California because so many people went there to look for gold. California had to be a free state since its constitution banned slavery. With the Compromise of 1850, California was admitted to the Union as a free state. However, the Fugitive Slave Act was passed. It required northerners to help capture slaves that had escaped to the North. This led to increased tensions between the North and the South.

Both sides eyed the rest of the Mexican Cession as an opportunity to get more new states on their side. While popular sovereignty would be used to determine if the new states would or would not have slavery, both sides would ultimately push hard to either ban or allow slavery in the new states that would eventually join the Union.

As a result, tensions increased between the North and the South.

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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.

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