Adventures of Rusty (1945)

Fearing that his recently-acquired step-mother, Ann Dennis, is competing with him for his father's affections, and saddened by the death of his dog, young Danny Mitchell seeks consolation in the companionship of a ferocious, Nazi-trained police dog, Rusty, brought to the U.S. by a returning WWII-veteran. The step-mother, with tender understanding, eventually wins Danny over while Danny pacifies his new dog.