This is a series I was planning on reading, at some point:
A vessel from a relatively unstable galactic federation(?) study study earth in passing during the 18th century. Deeming the humanity unstable and violent they decide to subjugate the race by sending in the youngest and most militant species to have join their federation. When this reptilian(?) race arrive at earth in the late 20th century they are shocked at the level of technological advancement and decide to take harsher measures including wiping out a fixed percentage of the human race. Surprise surprise - to them - the earthlings refuse to surrender and resist to the bitter end. Unprepared by the seeming ferocity and illogical nature of the counterstrike the aliens make a last-stand intending to deploy bio-engineered virus designed to wipe this abomination of a race from the galaxy. As luck would have it humanity instead manages to rub the invasion force out of existence. At this point the federation's worst nightmare is realized: an extremely violent race is in possession of highly advanced technology with an axe to grind.
I discovered this series a few months ago, and to the best of my recollection it is relatively modern (1990s-).