Showing Out of the Dark by David Weber

Earth is conquered. The Shongairi have arrived in force, and humanity’s cities lie in radioactive ruins. In mere minutes, over half the human race has died.

Now 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 the scattered survivors without getting killed.

His chances look bleak. The aliens have definitely underestimated human tenacity - but no amount of heroism can endlessly hold off overwhelming force.

Then, emerging from the mountains and forests of Eastern Europe, new allies present themselves to the ragtag human resistance. Predators, creatures of the night, human in form but inhumanly strong. Long Enemies of humanity . . . until now. Because now is the time to defend Earth.