The power grid is every advanced society's greatest weakness. It doesn't have to be a solar flare, locations of power lines aren't secret, and they're so long they're impossible to guard. It would be impossible even in my tiny country where you can't walk 5km straight without running into at least a small village; in a country as vast as the United States? No way, you'd have to have armed drones patrol everywhere 24/7, and it still wouldn't help if someone just drove a van full of explosives to the base of a tower and blew himself up for Allah or whatever.

It's something I've been thinking about for a long time, actually. Why do terrorists fly planes into skyscrapers and mow people down with trucks of peace, when they could plunge entire countries into months of complete anarchy and cause untold damage in terms of both human lives and the economy by knocking down a couple UHV towers with car bombs made out of readily available chemicals? Really gets those neurons firing, doesn't it.

(For any law enforcement agents reading this, I'm in no way suggesting anyone do this.)