[Bubba Ho-Tep]

Indeed, four states (Virginia, North Carolina, Tennessee and Arkansas) resisted joining the Confederacy until invading union armies crossed their borders.

Wrong. Those states seceded in reaction to Lincoln calling for troops after the south fired on Ft. Sumter, and long before any US forces stepped foot into their territory. Of course, this was exactly the reason that the confederate states chose to fire on Sumter in the first place, to push the wavering states into their camp.

[Vigilanteman]

The firing on Ft. Sumter was a South Carolina problem. It didn't bring the four northern Confederate States into the fold. Lincoln's reaction of calling for a full scale invasion of the south was what brought them into the fold.

Had Lincoln chosen a measured response, such as sending a contingent of marines to invade and recapture the island on which Sumter was located or even to occupy Charleston's harbor area, I doubt you would have gotten the same response.

Substantial areas of the south (what is now West Virginia and populations on both sides of the Appalachians, especially eastern Tennessee) didn't ever warm to the Confederacy, or did so only late in the conflict in reaction to the Union Army's deprivations.

The first major shooting battle of the war was a direct reaction to the Union Army's invasion of Virginia.