About 70 years ago Sci Fi author Robert Heinlein wrote a short story "The Roads Must Roll". The premise was that an extremely elaborate system of moving "roadways" were developed that allowed quick transportation from and to anywhere. A group of what we would now call terrorist blew up the power to some of the major roads. This caused a collapse of those roads of course but it also caused an overload of other roads that were feeders to those roads causing havoc there. The point is that it would not be necessary to shut down all electrical grids, just the major ones which would then cause overloads on the remaining grids.
In my humble opinion a cyber attack is just as much an act of war as was the attack on Pearl Harbor or the World Trade Center Twin Towers. Both would accomplish the same result, a destruction of another nation's economic or social structure. The fact that one required many to die and the other does not is not relevant since the end result is the same. Perhaps, as we become increasing tied to the Cloud and computers in general a cyber attack could be far more destructive than bombs and bullets.
https://en.wikipedia.org/wiki/The_Roads_Must_Roll