agreed, these carbon fibers would have to be hundreds of feet long in order to do any damage, and they would have to stay straight (or reasonably straight) in order to short out anything.
I think any dunce with even a little knowledge of tactics will realize that any major conflict is going to start with a cyber attack.
Not sure you understand how carbon fibers work: Kinda like graphite dust... conducts electricity all over the place. Imagine the dust in your PC is electrically conductive... how much damage would it do to the main board... thus rendering the computer useless. Now expand the thought to an entire communications grid... All one has to do is short out C&C... and the hardware becomes useless.