His Master's Voice (1925)

Too tender rearing makes a timid youth of Bob Henley, architectural student. He and Jack Fenton are rivals for the love of Mary Blake. War comes and Bob goes to France, his dog Thunder accompanying a Red Cross unit. Bob "finds himself" and develops courage. Thunder goes over the top and rescues him when Bob is wounded while trying to explode a mine. The dog completes the job. Bob returns home a hero. Finding that Fenton has attempted to double-cross him, he thrashes him and wins Mary.