Resolves NA if China does not attempt to invade Taiwan (before 2040)

Resolves NO if China does attempt to invade Taiwan and Japan is not involved militarily

Resolves YES if China does attempt to invade Taiwan and Japan is involved militarily

Involved militarily: clear reports that Japanese forces attacked Chinese forces or vice versa

Chinese invasion: I think this is going to be obvious; there should be widespread consensus. An invasion does not need to actually take place*, so long as there is substantial evidence that a credible force to invade Taiwan's main island was being massed on the Chinese coast.

Generally speaking, while you can imagine various scenarios with limited military conflict between Taiwan and China that are not an invasion it's hard to imagine Japan being involved in anything short of that. However, I reserve the right to make judgements on whether a scenario constitutes an invasion or not. For this reason I will not bet in this market. If I am no longer active then market participants may appoint a neutral arbitrator if there is disagreement.

*we could imagine that US, Japanese or Taiwanese actions might prevent it from actually happening