This can't be emphasized enough. At the time, secessionists were very clear and public about their motivations. Just look up "Declarations of the Immediate Causes Which Induce and Justify the Secession of South Carolina from the Union". I believe other confederate states similarly explained their actions and motivations, and it was the threat to slavery that was foremost. So modern day apologists seem to think they know better what motivated 19th c. politicians more than those politicians did themselves. Interesting.
I have heard two interesting assertions made regarding secession.
- The Constitution made no provision for secession by a state or states. It only provided a means of amending itself.
- That a number of states would never have signed on to the Constitution if they thought they could never, later, withdraw from it, should they wish to do so.
I am not a historian, so I cannot opine on the merits of those assertions, but I do find them very interesting.