Before the civil war, the idea of the US as being a group of nation states with a weak federal government was a lot stronger then it is now. The very question of whether a state could secede had never been asked, and to this day there is nothing in the Constitution to forbid it.

The soldiers weren't fighting for slavery; they were fighting for the right of succession from the Union. It was Lincoln who refused to let them leave. Lincoln who declared martial law and arrested people for speak against the war. Lincoln who suspended habeas corpus.