Answer:
There were also negative effects of Manifest Destiny. This idea that it was their destiny to expand caused Americans to disregard the territorial rights of Native Americans, wiping out many tribes and causing a cultural divide, tension and wars.
Polk's embrace of Manifest Destiny and the acquisition of new territories inflamed tensions between abolitionists and slaveholders and heightened conflict between white settlers and Native Americans.
cubed While the North and South fought over whether the new states admitted to the Union were to be free states or slave states, the battles between the Plains Indians and settlers in Texas grew particularly vicious. Though Comanches put up an especially fierce resistance to the encroachment of white people onto their lands, they were ultimately vanquished and relocated to a reservation in Oklahoma in 1875.
Meanwhile, the debate over the Wilmot Proviso was one of the major events leading up to the Civil War. The proviso, which was strongly opposed by the slaveholding South, asserted that the Mexican-American War had not been fought for the purpose of expanding slavery, and stipulated that slavery would never exist in the territories acquired from Mexico in the war. Ultimately, Polk’s territorial expansionism, though aimed at national unity, wound up intensifying sectional conflict and further paving the road to civil war
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