This event led to secession of seven southern states.

Answer: The event that led to the secession of seven southern states was the election of Abraham Lincoln as the 16th President of the United States in November 1860. Lincoln was a member of the Republican Party, which had a platform that opposed the expansion of slavery into the territories of the United States. This stance was perceived as a significant threat by the southern states, where the economy and social structure were heavily dependent on the institution of slavery.

As a result, between December 1860 and February 1861, seven southern states seceded from the Union. These states were South Carolina, Mississippi, Florida, Alabama, Georgia, Louisiana, and Texas. They joined together to form the Confederate States of America, a separate government. The secession of these states eventually precipitated the American Civil War, as the Union (the remaining states) did not accept the legality of secession and was committed to preserving the United States as one nation.