The premise? Hundreds of years ago, alien scouts witness the massacre of French nobility at Agincourt, and they decide that the human race needs to be eliminated. They send the only intelligent race of predators to do the deed, expecting/hoping that both the humans and this predatory race will do each other severe damage.
The alien attackers use kinetic weapons ("drop the rock") to destroy most human militaries and governments, but are shocked when the humans don't surrender.
And it goes from there.
Because I found the ending disappointing, I find it hard to recommend the book, but the rest of it was a fascinating read, so it may be something you'd enjoy. Try scoping it out at the book store...