Ur The White Mountains av John Christopher (1922-2012)

"Beanpole said, 'Look.'
His voice did not generally tell one much, but there was alarm in it. I said, 'Where?'
'To the west.'
He pointed. In the far distance something moved. The familiar hateful shape heaved itself over the skyline, followed by a second and a third. They were a long way off yet, but the Tripods were coming. "

John Christopher
1967