Age of Miracles — John Brunner

When suddenly all the fissionable material on Earth was exploded, Earthmen had their first notice of the aliens' arrival.

And by the time the panic, death and chaos had been sorted out, reports were coming in about mysterious cities scattered across the face of the planet - huge areas of flickering light and awesome free energy, disorienting to human senses and impregnable to attack.

The question was: were they alien bases . . . or something else?