The Strange Invasion — Murray Leinster

The Gizmo Invasion was unique in space and time. Not even the dawn of the Atomic Age with its Earth satellite sequel had stirred men more profoundly, or posed a greater challenge. They were spawned in a vast wilderness ... and the war they waged against Man challenged the miracles of modern science. It was the deadliest kind of biological invasion ... because the Gizmos seemed indestructible and were not even animals in a strict sense. They lived only to kill . . . . Then, the whole fighting fleet of the United Nations is caught in Kreynborg's marvelous, unique trap, in "Invasion." "The Strange Invasion" was published in Satellite Science Fiction digest magazine in April 1958. "Invasion" was published in Astounding Stories of Super Science in March 1933.