Courage of Lassie (1946)

Bill (Pal) gets separated from his litter, making friends with the wild creatures until he's found and adopted by young Kathie Merrick (Dame Elizabeth Taylor). An accident separates him from her, and he's drafted into K-9 duty in the trenches until battle fatigue takes its toll and he turns vicious. And even though he finds his way back home, he may be condemned as a killer.