Q:

Despite all odds, humanity prevails against indominitably superior alien invasion [yet again]

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 took a quick look at WhyYouShouldDestroyThePlanetEarth, HumansAreSpecial and HumansAreWarriors but nothing popped out.
I discovered this series a few months ago, and to the best of my recollection it is relatively modern (1990s-).

A:

Could this be "Out of the Dark" by David Weber? Here's the blurb from the ebook ....The Galactic Hegemony has been around a long time, and it likes stability--the kind of stability that member species like the aggressive, carnivorous Shongairi tend to disturb. So when the Hegemony Survey Force encountered a world whose so-called "sentients"—"humans," they called themselves—were almost as bad as the Shongairi themselves, it seemed reasonable to use the Shongairi to neutralize them before they could become a second threat to galactic peace. And if the Shongairi took a few knocks in the process, all the better.
Now, Earth is conquered. The Shongairi have arrived in force, and humanity’s cities lie in radioactive ruins. In mere minutes, more than half the human race has died.
Master Sergeant Stephen Buchevsky, who thought he was being rotated home from his latest tour in Afghanistan, finds himself instead prowling the back country of the Balkans, dodging alien patrols and trying to organize scattered survivors without getting killed. And in the southeastern US, firearms instructor and former Marine Dave Dvorak finds himself at the center of a growing network of resistance—putting his extended family at lethal risk, but what else can you do?
On the face of it, Buchevsky’s and Dvorak’s chances look bleak, as do prospects for the rest of the surviving human race. But it may well be that Shongairi and the Hegemony alike have underestimated the inhabitants of that strange planet called Earth…

