Jap:
Jap was not considered primarily offensive; however, during and after the events of World War II, the term became derogatory. Nisei veterans who served in World War II were shunned with signs that read No Japs Allowed and No Japs Wanted, denied service in shops and restaurants, and had their homes and property vandalized. [Wikipedia]
According to Japan Talk, the term Jap was not offensive before the World War II.
Quoting from Japan Talk. (The text is uncopiable so I'll post screenshots).
