My great-grandmother's father was a pro-Union man from Tennessee and was forced to move from that state in the post-war hostilities. So I think he'd agree that the war was far more than an east coast problem, as well.
On my father's side of the family, his mother's father was severely wounded during the Battle of Lone Jack, Missouri, but cited for "keeping his gun hot in the face of the Rebel attackers!"
One of these men was active in the "western theater" and the other two in the "trans-Mississippi theater". The latter described individual may have thought himself a westerner since his folks moved from New England to Indiana where he was born. He lived in a pre-war area of Missouri where neighbors owned slaves. So they might have thought of themselves as "southerners" . . . .
It may seem really hard to determine today what was in the mind of folks nearly one-hundred fifty years ago. But with the turmoil in Missouri and Kansas (where it seems to me "the war" began), and the fact that the last Confederate general officer to surrender did so at Doaksville/Fort Towson in the Chcotaw nation (now southeastern Oklahoma), I'd have to come down on the side of "the American Civil War" was a national event -- not at all limited to the eastern shores of the country.