It Conquered the World (1956)

It Conquered the World is a 1956 American science fiction film about an alien from Venus trying to take over the world with the help of a disillusioned human scientist. It was directed by Roger Corman, written by Lou Rusoff (with uncredited contributions by Charles B. Griffith who didn't wish his name on the film), and starred Peter Graves,Lee Van Cleef, Beverly Garland, and Sally Fraser.
Dr. Tom Anderson (Van Cleef), an embittered scientist, has made contact with a Venusian alien with his radio transmitter. The alien wants to take over the world using mind control devices, but claims it only wants to bring peace to the world by eliminating emotions. Anderson agrees to help the creature and even intends to allow it to assimilate his wife (Garland) and friend Dr. Nelson (Graves). The alien then disrupts all electric power on Earth, including motor vehicles, leaving Dr. Nelson to resort to riding around on a bicycle.

After killing a flying bat-like creature which carries the mind control device, Nelson returns home to find his wife assimilated. She attempts to force assimilation on him with another bat, and he ends up killing her. By then the only people who are free of control are Nelson, Anderson, Anderson's wife and a group of soldiers camping in the woods. Dr. Nelson finally persuades the paranoid Anderson that he made a horrible mistake about the alien's motives, allying himself with a creature bent on world domination. When they discover Tom's wife took a rifle to the alien's cave to kill it, they hurriedly follow her. The monster kills Mrs. Anderson before the two doctors can rescue her. Finally seeing the loss of everything he holds dear, Dr. Anderson kills the monster himself, dying in the process.

Production:
The script was originally written by Lou Rusoff, but before it was finished his brother died and he had to leave for Canada. Roger Corman called in Charles Griffith to rewrite it two days before filming commenced. The creature design was an idea of Corman's. He thought that since the creature came from a big planet, it would have been designed to deal with heavy gravity and would be built low to the ground. Corman later admitted this was a mistake, saying the creature would have been more frightening if it was bigger or taller. When Beverly Garland first saw the creature she commented "That conquered the world?" and kicked it over.
From Wikipedia

IT CONQUERED THE WORLD
ROGER CORMAN (1956)
AMERICAN INTERNATIONAL PICTURES
69 MIN
USA