I find an interesting philosophical subject in that the last time states seceded from the union for misrepresentation the remaining states declared them in rebe

I find an interesting philosophical subject in that the last time states seceded from the union for misrepresentation the remaining states declared them in rebellion to the agreement and spent a million lives(on both sides) forcing them back into union. Why not just let them go? Today given the outcomes of said conflict, it is generally agreed that this was the correct thing to do. Are there guidelines for when a secession is a genuine need to preserve local representation and when it is a gross attempt to preserve local culture that right thinking people have deemed illegal and are trying to stamp out?