The Bugle Sounds (1942)

Sergeant "Hap" Doan, heartbroken that the Nineteenth Cavalry, in which he has served for so many years, is to be mechanized and replenished with twenty recent draftees, goes on a drinking spree. He rails about leaving the Army, but is there when the draftees arrive, ready to make good soldiers out of them. After his horse, Cantigny, is killed by the explosion of a tank that had been sabotaged by Nazi agents, he goes AWOL, and is court-martialed upon his return and given a dishonorable discharge. But he is under secret orders from his commanding officer to join the gang of German spies who have every reason to believe they can trust the disgruntled ex-sergeant..