During the Civil War, what was the reason for secession of the Southern States?

The Southern States had different opinions from the Northern States.

The Southern States wanted to keep

Slavery State-Based Rule, not a Central Government

The Northern States wanted the opposite. Those were 2 MAIN reasons why the Southern States left. There were MANY other factors to this as well, such as the Missouri Compromise line, that rule was broken when the Northerners got California and other land that passed that line. Then there was also Bleeding Kansas which fueled both side's anger. Many slaves escaped into the Northern States to become free, and the Southerners passed laws that punished any Northerners that aided this process, the result was, tensions rose.

Overall, there was so many factors to the secession of the Southern States that you would have to spend quite a bit of time and you would have to analyze each occurrence and see which had the greatest impacts.

The two main that I found were

Slaves South wanted more power to States, North wanted Central Government

Hope it was of use. Good luck!