Related questions on Metaculus:

With tensions in the Taiwan Strait at a boiling point, it has been reported that Japan is considering becoming militarily involved if an armed conflict breaks out in the region. This is notable as Japan has not fought in any wars since the end of World War 2.

This question asks: will Japan fight on Taiwan's side if Taiwan is invaded by China?

If China invades Taiwan before 2035, will Japan respond with military forces?

If China does not launch a full-scale invasion of Taiwan before January 1, 2035, then this question will resolve as Ambiguous. If China does invade Taiwan, this question will resolve as Yes if Japan responds with military forces within 365 days from of the beginning of the Chinese invasion.

In line with our main question, China will be considered to have launched a full-scale invasion of Taiwan if either of these occur:

The Associated Press and the New York Times both report that the People's Republic of China has launched a full-scale invasion of Taiwan.
At least 1,000 military personnel from the People's Liberation Army have been deployed to Taiwan for the purpose of putting the sovereignty of the main island under PRC rule.

Japan will be considered to have responded with military forces if there is an offensive physical attack on Chinese infrastructure, ships, military personnel, or civilians, in direct retaliation to the full-scale invasion, and this attack is ordered by the Japanese government or Japanese military leadership. Cyberattacks will not be considered offensive physical attacks, but covert operations to e.g. assassinate Chinese nationals, will count.