If you have never heard about an EMP weapon, it's time you learned! It is perhaps the largest external threat out there. A single EMP detonated over the United States could destroy all electronic circuits from coast to coast. No electricity, no running water, no refrigeration, no transportation - a scenario for death for over one hundred million Americans. Here's a great list of EMP articles and resources...
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http://www.futurescience.com/emp/EMP-Sitemap.html