Rocketship X-M (1950)

Astronauts (Lloyd Bridges, Osa Massen, John Emery, Noah Beery, Jr., and Hugh O'Brien) blast off to explore the moon on. Because of craft malfunction and some fuel calculations, they end up landing on Mars. On Mars, evidence of a once powerful civilization is found. The scientists determinted that an atomic war destroyed most of the Martians (Who suprsingly look like humans). Those that survived reverted to a caveman like existance.