The Regiment #4: The Regiment’s War by John Dalmas

The poverty-stricken planet of Tyss has only one exportable resource, its extraordinary soldiers, hired by the Confederation of Worlds to train other warriors in their own mystic fighting abilities, a talent that soon becomes essential as the Confederation is confronted by an invading force from another part of the galaxy, in an omnibus volume containing The Regiment, The White Regiment, and The Regiment’s War.