In her 1924 article, “A laboratory study of fear: The case of Peter,” Mary Cover

Mary Cover Jones used counterconditioning to help the child get over his fear of rabbits.
Counterconditioning involves pairing the conditioned stimulus with a stimulus that elicits a response incompatible with the unwanted response.
In this case, a snack of crackers and milk was paired with presentation of the rabbit (CS).
The snack produced pleasant feelings incompatible with fear (CR).
To prevent fear of the rabbit from generalizing to the snack, the rabbit was initially kept at a distance and only gradually moved closer to Peter.
Systematic desensitization is a variation of this procedure sometimes used for treating phobias in adults.