... Simply put, an Electromagnetic Pulse (EMP) attack would occur when a nuclear weapon is discharged at a very high altitude, the explosion impacts the ionosphere and Earth's magnetic field in such a way as to cause an electromagnetic pulse to rush down to the surface. That pulse then bakes just about every electronic device within a very wide geographic area. By some estimates, a single device detonated over Kansas could cripple the nation's entire technical infrastructure.

From the 2004 Executive Report by the Commission to Assess the Threat to the United States from Electromagnetic Pulse (EMP) Attack: ...