What significant concept did the Kansas-Nebraska Act introduce to the issue of slavery in new territories?

The Kansas-Nebraska Act introduced the concept of popular sovereignty, which allowed the settlers in the newly created territories of Kansas and Nebraska to decide for themselves whether or not to allow slavery within their borders. This approach marked a significant shift in addressing the issue of slavery, moving away from previous federal mandates or prohibitions.

The principle of popular sovereignty emphasized the idea that the people living in a territory should have the right to self-determination and to make decisions about their own governance, including pivotal social issues like slavery. This act was critical in intensifying the national debate over slavery, as it led to conflicts and violence in Kansas, famously known as "Bleeding Kansas," as pro-slavery and anti-slavery factions clashed over the future of slavery in the region. Through the introduction of popular sovereignty, the Kansas-Nebraska Act contributed to the worsening sectional tensions that would eventually culminate in the Civil War.