Trial by Fire — Charles E. Gannon

When reluctant interstellar diplomat and intelligence operative Caine Riordan returns from humanity's first encounter with alien races, sudden war clouds burst. With Earth's fleet shattered by a sneak attack and its survivors fighting for their lives, Caine must rely upon both his first contact and weaponry skills to contend with the non-humanoid enemy. And when the technologically-superior attackers sweep aside the solar system's last defenses, and traitorous corporations invite the invaders to land ‘security forces,' humanity fights back with its best weapons: cunning, inventiveness, and guts.

But as Earth hurtles towards a final trial by fire that is certain to scar its collective memory, Caine discovers that there may also be large and disturbing gaps in that memory. Clues point to a much earlier inter-species apocalypse, buried in humanity's own prehistory. Which raises a terrifying possibility: what if the aliens' invasion of Earth is not one of conquest, but preemption? And what if their harrowing memories of a long-past cataclysmic war makes them willing to do anything to keep it from reigniting?

Even if that means exterminating the human race.