The Flying Saucers Aliens are a humanoid species looking much like a frailer version of humans which developped an advanced technnological civilisation and achieved interestellar space travel technology. They had to flee their home planet and entire star system was destroyed when their sun went cold centuries ago. Claiming to be that those inside the saucer fleet orbiting the Earth are the last of their species, they tried to invade Earth in the late fifties after some a dozen of artificial satellites had been put into orbit by Project Skyhook. After launching an attack on Washington, Paris, London and Moscow, they were destroyed by a weapon that could negate their saucers' magnetic propulsion system.
Source: Earth versus the Flying Saucers, 1956, Directed by Fred F. Sears, from a screenplay by Curt Siodmak, George Worthing Yates and Bernard Gordon, supposedly based on the non-fiction book Flaying Saucers from Outer Space by major Donald E. Keyhoe