Invasion
The greatest threat to Earth’s defenses wasn’t alien—it was already among us.

Invasion by Murray Leinster - The whole fighting fleet of the United Nations is caught in Kreynborg's marvelous, unique trap.It was August 19, 2037. The United Nations was just fifty years old. Televisors were still monochromatic. The Nidics had just won the World Series in Prague. Com-Pub observatories were publishing elaborate figures on moving specks in space which they considered to be Martian spaceships on their way to Earth, but which United Nations astronomers could not discover at all. Women were using gilt lipsticks that year. Heat-induction motors were still considered efficient prime movers.Thorn Hard was a high-level flier for the Pacific Watch. Bathyletis was the most prominent of nationally advertised diseases, and was to be cured by RO-17, "The Foundation of Personal Charm." Somebody named Nirdlinger was President of the United Nations, and somebody else named Krassin was Commissar of Commissars for the Com-Pubs. Newspapers were printing flat pictures in three colors only, and deploring the high cost of stereoscopic plates. And ... Thorn Hard was a high-level flier for the Pacific Watch.That is the essential point, of course—Thorn Hard's work with the Watch. His job was, officially, hanging somewhere above the twenty-thousand-foot level with his detector-screens out, listening for unauthorized traffic. And, the normal state of affairs between the Com-Pubs and the United Nations being one of highly armed truce, "unauthorized traffic" meant nothing more or less than spies.But on August 19th, 2037, Thorn Hard was off duty. Decidedly so. He was sitting on top of Mount Wendel, in the Rockies; he had a ravishingly pretty girl sitting on the same rock with him, and he was looking at the sunset. The plane behind him was an official Watch plane, which civilians are never supposed to catch a glimpse of. It had brought Thorn Hard and Sylva West to this spot.