What I am saying is that it was Mr. Lincolns war - a war of choice. A war of choice like WWI, Korea and Vietnam.
The South was either going to gain its independence or parish, reabsorbed back into the US empire. The USA's 18+ remaining states were going to survive as the USA no matter what the outcome below the Potomac. The "preserve the union" meme was cause celeb in 1861. It seems a bit stupid to me but who am I to say. I think the Copperheads had it about right.
Well, on those grounds you could just as well say the Second World war was also a "war of choice" since neither the Japanese, Italians nor Germans ever seriously threatened the United States homeland.
And yet nobody today doubts WWII was necessary and existential.
Further, "choice" may possibly apply to the 11 states which more-or-less legitimately voted to secede, it certainly did not apply to those eight states -- Maryland, Pennsylvania, West Virginia, Kentucky, Missouri, Kansas, Indiana & Ohio -- which did not declare secession but were invaded by Confederate forces.
Especially "choice" did not apply to Kentucky & Missouri which Confederates claimed as their 12th & 13th states.
Nor did "choice" apply to those territories -- Oklahoma, New Mexico & Arizona -- claimed & invaded by Confederate forces.
Nor did "choice" apply to others -- California, Colorado & Vermont come to mind -- where Confederate guerillas operated.
So there was no possible way for the Union to avoid war short of abject surrender to all Confederate demands.
Oh, you might say: it was still "choice" to conquer & occupy Confederates, that wasn't necessary.
I'd say: just as it wasn't "necessary" to conquer & occupy Axis countries in WWII.
But nobody I know of today argues we shouldn't have done it or that the world isn't a lot better place because we did.