To challenge heaven / David Weber and Chris Kennedy.

Enhanced descriptions from Syndetics:

The third entry into the New York Times bestselling series, To Challenge Heaven brings another thrilling adventure from the masters of military science fiction, David Weber and Chris Kennedy.

In a universe teeming with predators, humanity needs friends. And fast.

We've come a long way in the forty years since the Shongairi attacked Earth, killed half its people, and then were driven away by an alliance of humans with the other sentient bipeds who inhabit our planet.

We took the technology they left behind, and rapidly built ourselves into a starfaring civilization. Because we haven't got a moment to lose. Because it's clear that there are even more powerful, more hostile aliens out there, and Earth needs allies.

But it also transpires that the Shongairi expedition that nearly destroyed our home planet ... wasn't an official one. That, indeed, its commander may have been acting as an unwitting cats-paw for the Founders, the ancient alliance of very old, very evil aliens who run the Hegemony that dominates our galaxy, and who hold the Shongairi, as they hold most non-Founder species, in not-so-benign contempt.

Indeed, it may turn out to be possible to turn the Shongairi into our allies against the Hegemony. There's just the small matter of the Shongairi honor code, which makes bushido look like a child's game. We might be able to make them our friends -- if we can crush their planetary defenses in the greatest battle we, or they, have ever seen...