On Monday July 5, 2021, Japan's Deputy Prime Minister, Taro Aso, said that Japan must defend Taiwan if China invades the island due to the "direct [PRC] existential threat" to Japan's security. Aso added "If Taiwan falls, Okinawa will be next. We must think about this seriously, and steadfastly prepare our defence might". Taiwan is just 110km off the coast from Japan's westernmost point, the populated island of Yonaguni.
An intervention of the large numbers of conventional Japanese naval (see submarines and surface fleet) aircraft and missile assets may be very helpful to Taiwan. This is partly because Taiwan's own assets might be early and quickly destroyed in a PRC conventional missile first strike. Also the US can only focus relatively smaller conventional naval-air assets (compared to the PRC's growing regional arsenal) in defence of Taiwan. The US nuclear superiority over the PRC would not be helpful as the US would not wish to go to nuclear war over Taiwan.
Japan has significant anti-invasion assets, but it also has a more moderate policy on protecting Taiwan than converyed by Aso on July 5th. After Aso's July 5 comments China's Foreign Ministry spokesman Mr Zhao Lijan predictably responded that Aso"harmed the political foundation of China-Japan relations”.
So on July 6 Aso retracted and retreated to Japan's long held moderate policy - any major tension between the PRC and Taiwan should be resolved through dialogue between PRC and Taiwan. Implicitly Japan's should stay out of any PRC-Taiwan fighting.
But if Aso has been a Japanese equivalent of one of China's new breed of Wolf Warriors his comments could be dismissed as exteme propaganda posturing. Aso is, instead, a very senior politician who has been Deputy Prime Minister since 2012, Minister of Finance since 2012, Minister of Foreign Affairs (2005–2007) and even Prime Minister (2008-2009). He has been in the Japanerse parliament (the "Diet") for 42 years, He is also a Liberal Democratic Party (LDP) politician like current Prime Minister Yoshihide Suga. So Aso is influential with the Prime Minister, foreign policy and most probably the closely interwoven Japanese defence policy.
SUBMARINES
As well as supportive words to Taiwan from Aso (even if retracted) Japan has been more quietly contributing to Taiwan's program to design and construct 8 submarines. Taiwan has has repeatedly flagged that its new submarines would be of teardrop design. but that may be a deception. I know of no countries who are still building teardrops. More likely Taiwan's submarines will have a more modern sub's cigar shape.
If Japan were strongly influencing the basic hull design then a Soryu cigar shape may be possible, especially if Taiwan's subs feature AIP for longer terms sitting on the seafloor, guarding the Taiwan Strait.
Many of Japan's own (soon to be) active fleet of 22 submarines could make a major contribution to destroying PRC invasion force subs and major surface vessels trying to cross the Taiwan Strait.
It will be interesting to see if Aso's hard-line tone against China, voiced on July 5th, is later persistently or at least sporadically repeated by Japanese politicians, admirals and generals.