But what would happen if major powers start a war? A Netsplit? Would a major power isolate himself from the world as a security measure against eletronic attacks? If yes, it would be a physical disconnection or a logical one?
The real world is less glamourous than a Tom Clancy novel. :)
Critical services are not on the internet in the first place and it is more useful as an information channel/propoganda tool than it is a risk of electronic attack.
I very much doubt that any major power would cut themselves off from the rest of the world. More likely is a great firewall like China already have in place and Iran used during recent troubles filtering the ideas that it's people are exposed to from the rest of the world and its perceived enemies.