Victory by Lester del Rey audiobook.
The earth is undefended. A ripe, juicy, rich plum just innocently waiting for one of the many new and warlike civilizations in the nearby universe to reach down and pluck to plunder and rape. Well... undefended can mean many things. In this case, humans have colonized many worlds and found many surprisingly human like races among the stars. But when these human colonies are threatened and even viciously attacked by really alien civilizations, earth does nothing to help them. Earth, fat, rich earth sits back and lets her own colonies be practically wiped out, claiming a neutrality that seems a thin veil for cowardice and deceit. How this happens and the story behind the story is what this exciting story is all about.