Not quite correct. It would be more accurate to say that Unionists defined secession as evil from day one and secessionists obviously disagreed. Some still do, as can be seen by many comments in this thread. What was settled by the war was not whether secession was evil, that is something that can still be reasonably debated, but which side of the disagreement would prevail in practical terms.
While in my opinion secession was an evil and stupid policy, I'm not (quite) arrogant enough to classify my opinion as fact.
I wonder how many southerners thought that "the army would never turn its guns on the people" - or on the people's militia??
You should recognize the fact that the initial stages of the war were not "the southern militia" versus the "Federal army," in the sense we might view a similar conflict today.
For all practical purposes, there was no federal army in 1865. The Virginia militia alone had 185 regiments and dramatically outnumbered the federal army of 15,000 men, which was scattered all over the continent anyway. And of course, a disproportionate percentage of the best and brightest leaders of the federal army deserted to fight for the rebels. The conflict in its initial stages was almost entirely between rebel and loyal formations of "the people's militia."
Of course, by the next year the volunteer federal army had become a thoroughly professional force, but then the same was true of the Confederate army.
“And of course, a disproportionate percentage of the best and brightest leaders of the federal army deserted to fight for the rebels.”
Wow, you sure make no bones about what side you support. For the record, those leaders resigned their commissions (which was their right as officers) and did NOT desert.