Final answer:
Popular sovereignty, as used in the Kansas-Nebraska Act, referred to the principle that settlers in the territories could decide whether to allow slavery, leading to severe sectional conflict and foreshadowing the Civil War.
Step-by-step explanation:
Popular sovereignty was a principle applied to the Kansas-Nebraska territories under the Kansas-Nebraska Act of 1854. This act, introduced by Illinois Senator Stephen A. Douglas, determined that the settlers of the territories would decide for themselves whether to permit slavery. This essentially nullified the Missouri Compromise line, aggravating sectional tensions and contributing to the outbreak of the Civil War.
The doctrine of popular sovereignty meant that the residents of a territory had the right to decide the issue of slavery based on majority rule. In practice, this led to violent clashes in Kansas, known as "Bleeding Kansas," as pro-slavery and anti-slavery forces battled to influence the outcome of local elections. The Kansas-Nebraska Act thus sowed further division between the North and South and was a significant step towards the dissolution of the Union.
The Kansas-Nebraska Act also had major political ramifications, including the demise of the Whig Party and the birth of the Republican Party, fundamentally transforming the American political landscape.