“We don’t have experience with losing the infrastructure in a country with 300 million people, most of whom don’t live in a way that provides for their own food and other needs. We can go back to an era when people did live like that. That would be — 10 percent would be 30 million people, and that is probably the range where we could survive as a basically rural economy.”

U.S. House of Representatives, Hearing, “Threat Posed By Electromagnetic Pulse (EMP) Attack” Committee on Armed Services (Washington, D.C.: July 10, 2008), p. 9.

This is the quote that is the cornerstone of this article. Hypothetically, if an EMP destroyed enough stuff to the point that we couldn't get everything back up and running in a reasonable amount of time, we'd suddenly not have the infrastructure to support even a majority of living citizens. Also, in that hearing, it was acknowledged that this is unprecedented and that nobody would really be able to say what exactly would happen, which led to this 'worst-case' guess.

It's incredibly unlikely and probably almost outside the realm of possibility, but if the entire power grid and most electronics themselves got knocked out, we'd be in the 1800s for a little while. Farmers could barely farm, let alone get food moved. People wouldn't have electricity, by extension water, and probably no money to boot. That type of thing. Worst-case, it'd be the apocalypse.