Shooting clarifies things. Once the shooting started it became impossible to remain “neutral,” although Kentucky made a less than successful attempt to do so. The Upper South was faced with only two alternatives, fight on the side of or against the CSA. Not surprisingly, they went with their brothers in the Deep South.
I didn’t say the attack on Sumter was dastardly. I do say, though, that it made it a great deal more difficult for the CSA to portray itself as having the war forced upon it. This had major positive impact on northern public opinion, and in particular made it much easier for northern Democrats to support the war effort without losing face. They were fighting a war of defense, not of offense.
Prior to Sumter, public opinion in the North was deeply divided about whether force should be used. After Sumter, it was (temporarily) almost unanimous that war was necessary. This was absolutely critical, because the federal government had no mechanism by which it could force the northern people to fight. Without popular and state government support Lincoln could issue proclamations, but he most certainly could not raise an effective army.
It also did significant damage to the southern cause in Europe.
On the other hand, the accession of the Upper South states to the CSA almost doubled both its military manpower and its industrial capacity. Without these assets it could certainly not have fought for four years.
There’s an excellent discussion of these issues in American Iliad.