Tens of thousands of cyber attacks on the power grid are troubling, though so far they have rarely caused damage. More alarming is news of an old-fashioned armed attack on a physical location that proved the vulnerability of the grid.

Last April, a nighttime attack destroyed a power substation in San Jose, Calif., the center of Silicon Valley. The attackers had a good understanding of the facility and how to destroy it. They broke into an underground vault off Highway 101 and cut fiber-optic cables. Then they fired on the substation...