Which primary source document could help a historian understand why the US Civil War started?

The primary source document that could help a historian understand why the US Civil War started is a Charleston Mercury editorial published in 1860 because it contains factual accounts of the events during that time.

geography

this is because warmer climates often don't get people sick from the flu and regions where it was often colder more people received the flu. hope this if you need more reasoning i am here to