In a recent letter to investors, billionaire hedge-fund manager Paul Singer warned that an electromagnetic pulse, or EMP, is "the most significant threat" to the U.S. and our allies in the world. He's right. Our food and water supplies, communications, banking, hospitals, law enforcement, etc., all depend on the electric grid. Yet until recently little attention has been paid to the ease of generating EMPs by detonating a nuclear weapon in orbit above the U.S., and thus bringing our civilization to a cold, dark halt.

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