THE BERSERKER By CHARLES V. DE VET - https://www.gutenberg.org/cache/epub/64561/pg64561-images.html

 

Approximately twenty years earlier—in 1950—the aliens had somehow made their "door" between the worlds; that "door" which never appeared twice in the same spot. At first they had been content to come in, circle their noiseless vessels through the air as they observed the Earth, then return through their shifting "door." They had refused all contact. Then gradually evidence began to come in that they were raiding undefended areas, abducting men and stealing property. Their depredations increased through the years until eventually they constituted a major menace.