U.S. President Donald Trump signed a new executive order on Tuesday, mandating a government-wide effort to harden critical elements of the nation’s infrastructure against the possibility of an electromagnetic pulse (EMP) attack.

Put simply, an electromagnetic pulse is a short burst of a specific kind of radiation that’s most commonly released by a nuclear detonation. EMPs represent a particularly significant threat to the United States from nations with underdeveloped nuclear programs such as North Korea and Iran, as the weapon doesn’t need a particularly hardened reentry vehicle in order to function. Instead, the nuclear warhead is detonated high above the target area in low earth orbit.

Electromagnetic radiation comes in three types of pulses (commonly called E1, E2, and E3) that effect everything within line of sight of the blast, overloading electronic devices, disrupting the flow of electricity, and generally throwing the affected area back into the Stone Age.

The Washington Examiner quoted EMP expert Dr. Peter Vincent Pry, author of the report “Nuclear EMP Attack Scenarios and Combined-Arms Cyber Warfare.” In it, Pry details the devastation of an EMP attack.