The event that caused the Southern states to secede was the election of Abraham Lincoln as the President of the United States.
The presidential election of 1860 was held on November 6th. By December 20th, South Carolina became the first state to secede. Many others followed in January. The reason for this was that the South felt that Lincoln’s election meant the end of slavery. While Lincoln was personally opposed to slavery, he felt that the US government did not have the right to ban it. However, Southerners were certain that he would move to end slavery. They were also unhappy about the fact that Lincoln was elected even though he did not get any votes in the South. They felt that this showed that they had lost all power in the country and that the North would eventually ban slavery. Because they felt that they had lost power and would have abolition imposed upon them, the seceded. The event that was the immediate cause of this was the election of Abraham Lincoln.
It was the election of Abraham Lincoln as president which caused the Southern states to secede. Lincoln was elected president on November on November 6, 1860, and secession began just a few weeks later: South Carolina went first, on December 20, followed by Mississippi on January 9, 1861, and then by Florida on the next day.
For the Southern states, secession from the Union was about saving slavery. They felt that Lincoln wanted to bring this practice to an end and they were prepared to take extreme action to prevent this from happening. By the time of Lincoln's inauguration, for instance, seven of the Southern states had already seceded from the Union and war was just one month away.
Lincoln's personal views on slavery were, therefore, one of the most important causes of the Civil War.