Ralph Compton's : clarion's call / a novel by Robert Vaughan.

Enhanced descriptions from Syndetics:

He followed the call of his country-straight into the frontier's bloodiest battle…Quinn Pendarrow is looking forward to a new chapter in his life. Obtaining an honorable discharge from the U.S. Army, he sets off to marry his sweetheart-only to discover that while he committed himself to his country, his intended committed herself to another…With nowhere else to turn, Quinn re-enlists as a civilian member of the Trains Company, responsible for the delivery of military supplies to units in the Dakota Territory. He never expects to meet a woman under these circumstances-yet he soon encounters the talented and beautiful Kate, daughter of the respected painter P. D. Mackenzie. The Mackenzies are touring the West to paint the Plains Indians-but they don't know the first thing about the dangers of the frontier. Even as Quinn undertakes to protect Kate, he's called upon to aid General Custer's forces for an upcoming battle. A battle that will go down in history-and change Quinn's life forever…