What triggered a Civil War between the Union and the Confederates?

The answer generally revolves around slavery and a difference in opinion on the role and spread of slavery throughout America. There are nuances within this though, with issues of arguments over states rights vs the federal governments, economic considerations, as well as actions taken by individuals such as the Dred Scott Case, Tom Brown's action and the election of Lincoln. The eventual trigger is Six States leaving the Union and the Confederates opening fire on Fort Sumter.