In the years leading up to the Civil War, debates over state sovereignty versus federal authority became particularly contentious. The passage of the Fugitive Slave Act in 1850, part of the Compromise of 1850, and the Kansas-Nebraska Act of 1854, which allowed new territories to decide on the legality of slavery, intensified these conflicts. To further understand the underlying causes of the Civil War, which of the following movements or developments most directly challenged the existing legal framework regarding slavery and significantly contributed to the sectional tensions?