The Regiment: a Trilogy (Baen Books Megabooks)

The planet Tyss is so poor that it has only one exportable resource: its fighting men. Each year three regiments are sent forth into the galaxy. Hiring their services is very expensive, but well worth it, for the Tyss secret training makes their soldiers into mystic warriors, irresistible in battle. And when they offered to train soldiers from off their planet, the Confederation of Worlds jumped at the chance, using their personal selection technology to pick the greatest potential warriors out of their planets-wide database of psych profiles. What they didn't know was that the new warriors were soon going to be necessary for the Confederation's very survival - because an invading force from another part of the galaxy is approaching with superior firepower and more advanced technology, and only the secret training of Tyss can give even a slight hope of stopping the invaders.