Not the Way the Smart Money Bets — Michael A. Stackpole

From Book 1: They are coming!

From beyond the periphery comes a threat. A military force of unkown origin.

Nothin we have can stop them. Their power, speed and ferocity are unparalleled. Some of the finest warriors and ablest units have challenged them and failed. No force from the Inner Sphere has faced them and won.

The Clans! An alien juggernaut whose sole reason for being is battle. A race that selectively breeds itself for combat. Humanity's only hope is an alliance of mortal enemies. The Federated Commonwealth and the Draconis Combine, interstellar empires at war for 300 years, must now stand side-by-side or face certain destruction.