One high-altitude nuclear detonation will create a massive electromagnetic pulse that can bring down the U.S. national power grid and keep it down for many months, perhaps a year or longer.
Americans would instantly find themselves without running water, food, refrigeration, lights, phones, functioning toilets and sewage systems, air conditioning and heating, transportation (no gasoline), and without access to their bank accounts or medical services.
A nuclear high-altitude electromagnetic pulse will damage and destroy the modern electronic devices required to run U.S. critical national infrastructure; it will also disable the Emergency Power Systems and active Emergency Core Cooling Systems at dozens of U.S. nuclear power plants, causing the meltdown of their nuclear reactors.
All legislative efforts to protect the grid and critical national infrastructure from electromagnetic pulse have been blocked by the electric and nuclear utilities, as well as by the Nuclear Regulatory Commission.