[vitaminj]

If you look at the major blackouts around the world over the last 10 years, a large chunk of them were caused by a variation on a similar theme:

- The network is heavily loaded pre-blackout (most large networks are these days)

- A major interconnect trips out (e.g. from a tree strike or protection maloperation)

- The other line is out of service (e.g. for maintenance) or is taken out by the same event (e.g. by storms)

- Power flow is redirected through other weaker interconnects causing voltage instability

- Cascading voltage collapses take out the network

Examples: India 2012, European blackout of 2006, Indonesia 2005, northeast blackout of 2003, Italy 2003 (a bit different because the operators cocked this one up as well).

The point is that you can target one or two major transmission lines (usually in the middle of nowhere) and bring down the system. A coordinated attack on several major interconnects could really cause some damage.