Something interesting came through on the Sankei Shimbun yesterday, to the effect that Japan was joining AUKUS:

Kantaro Komiya @KantaroKomiya

Australia, Britain and the United States have asked Japan to join the security pact AUKUS, Sankei newspaper reported, citing multiple government sources. AUKUS expects synergy with Japanese technologies on areas such as hypersonic weapons and electronic warfare, Sankei said.

産経ニュース @Sankei_news

＜独自＞ＡＵＫＵＳ参加、米英豪が日本に打診　極超音速兵器など技術力期待 https://t.co/TebsV3rm4c 日本政府内ではＡＵＫＵＳ入りに積極的な意見がある一方、米英豪３カ国とは２国間の協力枠組みがあるため、参加の効果を慎重に見極める考えもある

April 12th 2022

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About thirteen hours later came the official denial:

Kantaro Komiya @KantaroKomiya

Japan's top government spokesperson denied the Sankei report on AUKUS's approach to Japan. "There is no fact" that the trilateral security pact has asked Japan to join, Chief Cabinet Secretary Hirokazu Matsuno said in a Wednesday press conference. 米英豪がＡＵＫＵＳ参加を日本に打診した事実はない＝官房長官松野博一官房長官は１３日午前の記者会見で、米国、英国、オーストラリア３カ国の安全保障枠組み「ＡＵＫＵＳ（オーカス）」に日本が参加を「打診された事実はない」と述べた。jp.reuters.com

April 13th 2022

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None of this is particularly conclusive. If Japan has been invited to join AUKUS, the announcement almost certainly wouldn’t be coming via a scoop in a mid-circulation regional paper whose readership is exceeded by that of Tokyo Sport. That also doesn’t mean Sankei got it wrong. Japan in AUKUS makes sense on multiple levels — which is why so many people on Twitter immediately accepted the news as fact — and the question of its participation seems more like one of when than whether.