According to Japan Talk, the term *Jap* was not offensive before the World War II.
 
Quoting from *[Japan Talk][1]*. (The text is uncopiable so I'll post screenshots).

[![enter image description here][2]][2]
[![enter image description here][3]][3]


  [1]: https://www.japan-talk.com/jt/new/is-the-word-Jap-derogatory
  [2]: https://i.sstatic.net/97Baz.jpg
  [3]: https://i.sstatic.net/KSdLg.jpg