The correct answer to this open question is the following.
The event that turned the tide prior to the Civil War was the election of 1860. Less than a month after this, South Carolina and other states declared their intent to secede from the United States.
In the United States election of 1860, Republican candidate Abraham Lincoln, former Senator from Illinois defeated Sothern Democratic Party candidate John C. Breckenridge. Hannibal Hamlin was the vice president in Lincoln's formula. When this happened, the nation was merely divided than never before, causing the southern states to express their intention to secede from the Union. This was one of the causes of the Civil War that started in April 1861.
