For the last couple of years, the meme of an EMP attack against the United States has been an integral part of the thoughtbase of the prepper community. So the idea goes, the next major attack by a foreign power will involve not the bombing of a major city but bombardment with an electromagnetic pulse (local mirror, snapshot taken 20170310 @ 2030 hours PST8PDT). Due to the fact that "electromagnetic" is kind of a loose term, sometimes they mean an actual magnetic field, sometimes they speak of a microwave burst (which means that you've got bigger problems than your electronics getting fried - humans are mostly water, after all), sometimes they mean RF, and sometimes they mean some other unspecified thing. At any rate, the pulse emitted is enough to fry all major electronics, knock out the power grid, and generally return the country to a hunter-gatherer mode of existence for the forseeable future. Just how this happens is never really explained but the answer can be determined with basic physics. Electricity and magnetism are two sides of the same coin: Where you have one, you have the other. Pass a powerful enough magnetic field through a long enough wire and it might generate enough voltage to blow out the components soldered to it. Do that to enough electronic devices in the area, and all the equipment goes down. Seems simple enough.
So, what's actually the score here?
Fifty-five years ago, the United States government wanted to find out what would happen if somebody popped off a nuke in space. So, the initiated a project called Starfish Prime, in which they detonated a 1.4 megaton nuclear device 240 miles above the surface of the Earth, a distance which is on the low end of low earth orbit. The detonation created an artificial aurora that was seen in the sky for thousands of kilometers around, in addition to scattering fallout in LEO and the upper atmosphere. To be fair it was probably only a little fallout, relatively speaking, because it was only the remnants of the nuke itself and not the vaporized debris one would expect of a terrestrial detonation. It was observed by the project's scientists that the orbital detonation generated an electromagnetic pulse that briefly disrupted electrical power on the ground hundreds of kilometers around where the center of the blast sphere was. It was later discovered that Telstar-1, the first comsat launched into orbit, was damaged by the radiation. In Hawaii, the power surges were such that street lights blew out, knocked out telephones, and caused radio blackouts. Physicists later determined that the burst of electrons loosed by the detonation were trapped by the Earth's geomagnetic field and didn't return to a low-energy state for several months. This had the net effect of interfering with radio propagation for about as long, making communications difficult.
Seems legit so far.