I really do not think there is a "proof positive" for either. The Union had their reason, the Confederate States had their reason. The individual soldier in either of them was just a tool to get a job done. I’m not sure anyone really cared what they thought as an individual. That hasn’t changed since time began. Officers or Leaders give the orders and those under them follow them. If they wish to return home, they try to do a good job. Sort of applicable to the soldier today too.
McPherson, in my own, uneducated opinion, is one of the least "fair and balanced" authors or 'experts' around.
We have hashed an rehashed the States Rights issue over and over. It was probably the last thing a Confederate soldier discussed when writing home but it was very much the reason for there to be a Confederacy of States. In the time the CW was fought, many of those who were fighting had Fathers or Grandfathers who fought in the Revolutionary War or the War of 1812. It was better understood, in that time I think, than it could be now. One could possibly view the CW as a failed experiment. That attempt pretty much sealed the Union and I doubt there will ever be another attempt to bow out of it by a state. That said, the individual state does have the right to make some decisions. This is an issue today. We see it on the news daily. The more I see it in today’s light, the better I understand it 150 years ago and I also am feeling like they may have been on to something that truly wasn’t that clear to me before.. At least they tried. It just didn’t work out.
If one is in an unhappy marriage, they can get a divorce. If one is in the Union of the United States, nope, ain’t gonna happen. Irreconcilable difference wording will not fly if one is a state.