MIL-STD-188-125-1 is the United States military standard that specifies the protection requirements for fixed ground-based command, control, communications, computers and intelligence facilities against the electromagnetic pulse produced by a high altitude nuclear detonation. The threat the document addresses is distinctive: an explosion at high altitude produces a brief, intense electromagnetic field over a continental area, which drives large currents into any long conductor such as a power line, a telephone cable or an antenna feeder, and those currents then enter the facility and threaten the electronics inside. Because the facility may be required to continue operating, the protection is not a matter of simply disconnecting cable but of shielding the structure, installing protective devices at every penetration and proving that the resulting system attenuates the threat by the required margin. The standard defines the threat environment, the required hardness level, the design and construction measures, and a verification programme that combines analysis with pulse current injection tests.