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
John Brunner (1934-1995)
John Brunner was a prolific British SF writer. In 1951, he published his first novel, Galactic Storm, at the age of just 17, and went on to write dozens of novels under his own and various house names until his death in 1995 at the Glasgow Worldcon. He won the Hugo Award and the British Science Fiction Award for Stand on Zanzibar (a regular contender for the 'best SF novel of all time') and the British Science Fiction Award for The Jagged Orbit.