Since the excellent response to a previous question, can anybody help with this one? Read in hardcover somewhere around '60 to '62, it was a space-opera checked out of my library's juvenile section.
Earth has begun venturing out to the stars, and has met and is anticipating an invasion by a conquering alien force known as the Saurians. Neighboring human-inhabited star systems are being gobbled up, but archaeology and language proves the remaining new allies are related to earth. Eventually, it is learned that earth had spawned colonies in a dim past and been attacked, defeated and isolated to the ice ages, maintained by remote machinery on an outer planet (Mars, or a moon of Jupiter or Saturn). The fear of humanity now is that the previous conquerors were also the Saurians.
At the end it's revealed from the (defeated) Saurians that they were not the original conquerors, nor did they know where they came from or went to. I vaguely remember the originals might have been referred to as the master-somethings or something-masters (I was 9 -12, after all). I also remember I had gone through a lot of Lester Del Rey at the time.
Ring a bell, anyone?