A Boy, a Girl and a Dog (1946)

During World War II, a young boy and girl, living with their respective families in an apartment house that had restrictions against pets, adopt a lost dog and hide it in a vacant apartment, which may have been the only vacant apartment in the United States at the time this movie was being filmed. A burglar breaks in and the apartment is damaged when the dog and crook have a tussle. This blows the dog's cover, but the kids enlist him in the K-9 Corps, and the dog distinguishes himself in the WWII Italian campaign.