Tactics of Mistake | Gordon R. Dickson
It's obvious that Cletus Graeme--limping, mild-mannered scholarly--doesn't belong on a battling field, but instead at a desk working on his fourth book on battle strategy and tactics. But Bakhalla has more battlefields than libraries, and Graeme sees his small force of Dorsai--soldiers of fortune--as the perfect opportunity to test his theories. But if his theories or his belief in the Dorsai lead him astray, he's a dead man.
Tactics of Mistake | 
