How did the issue of slavery continue to divide the nation after the Mexican War?

Slavery continued to be a highly contentious issue, to divide the nation and feed bitter disagreeements.  

After the Mexican-American War ended in 1848, abolitionists and slavery supporters in politics and Congress clashed over the future status of the new states that would be created from the newly won territories. Abolitionists opposed the admission of new slave states in the Union; slavery partisans wanted to give the new states the freedom to choose by themselves. Both camps reached a new legal-political compromise in 1854 with the Kansas-Nebraska Act, which replaced the Missouri Compromise of 1820.  It helped keep the balance. Nevertheless, the infamous Dred Scott Supreme Court ´s decision of 1857 that declared black people were not US citizens incensed abolitionists. In 1861, the Civil War erupted.

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