Which event triggered the secession of Southern states from the Union?

The election of Abraham Lincoln in 1860 is recognized as the event that triggered the secession of Southern states from the Union because his platform opposed the expansion of slavery into the new territories. Southern states viewed Lincoln’s election as a direct threat to their way of life and institution of slavery, which they saw as essential to their agricultural economy.

Lincoln's victory, achieved without any support from the Southern states, signified that the concerns and interests of the South were no longer represented in the federal government. As a result, several Southern states began to secede from the Union, starting with South Carolina shortly after the election, ultimately leading to the formation of the Confederacy.

The other options, while significant in the context of sectional tensions leading up to the Civil War, did not have the immediate impetus on secession that Lincoln's election did. For instance, the Missouri Compromise and the Kansas-Nebraska Act reflected the ongoing struggle over the expansion of slavery but did not directly lead to the decisive action of secession. The Dred Scott decision further inflamed tensions but occurred in a context where secession had not yet taken place.