By saying this, the author is trying to emphasize two things.
First, the author is trying to emphasize that the South's motives were not all that bad and secession was not really its fault. The South (the author is saying) did not want to secede. It would have loved to stay in the Union but it was driven out by the policies of the North. This casts the South in a somewhat more positive light.
Second, the author is trying to emphasize that secession came about after a long process. This was not something that the South just did in a rash manner. Instead, secession only came about as a last resort after many other things had been tried and had failed to resolve the issues between North and South.