Perhaps the most devastating thing that could happen in any developed country would be widespread catastrophic damage to its electric power grid. Nearly everything in an industrialized nation relies on electricity to function. Without it, normal water supplies, sewer systems, and communication services are cut off. Furthermore, things like food and transportation are quickly affected when power is down for extended periods.
A severe electromagnetic pulse (EMP) or geomagnetic disturbance (GMD) event could take the power grid down for months, and possibly even for years. An EMP is a very intense pulse of electromagnetic energy, typically caused by the detonation of a nuclear bomb or other high-energy explosive device.
