Holding the Line — Rick Shelley

SPEC OPS SQUAD
After eons of searching, humanity finally learned it was not alone -- that there were many intelligent alien races among the stars. By joining the Alliance of Light, Earth had discovered allies with whom to explore the universe -- and an enemy that would never rest until the Alliance lay in ruins....

Sergeant Bart "Dragon" Drak, just back from fighting on the planet Dintsen, is given a daunting new task. Assigned to an experimental regiment as a Spec Ops Squad leader, he must try to Create a unit made up of every alien race in the Alliance. And with all the bad blood among the aliens, training these troops to fight as a team might be impossible.

But before the training is completed, Drak gets the news that the Ilion Federation is making a push toward the core worlds, which would spell doom for the Alliance. The orders: Drak must return to the hellish battleground of Dintsen with his regiment, and hold off the Federation tide to the last manor alien....