Red ellipse marks the Tsugaru Strait (Map courtesy Wiki commons)
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Some years ago, Japan adjusted the territorial waters limits of some strategic straits between major Japanese islands to make the passage of potentially nuclear armed USN warships and submarines legal under Japan's "no nuclear weapons on our territory" laws.
One such strait is the Tsugaru Strait which separates Honshu and Hokkaido in northern Japan (see map above). This Strait connects the Sea of Japan with the Pacific Ocean. For further details on the Tsugaru Strait’s territorial situation see Wiki here.
So potentially nuclear armed USN warships and USN submarines have been legally transiting the Tsugaru Strait for years.
The problem is potentially nuclear armed warships of other nations also have the right to transit the Strait.
Japan Today on October 19, 2021, reported:
“Chinese and Russian naval vessels on [October 18, 2021] passed together through the Tsugaru Strait...for unknown reasons, the [Japanese] Defense Ministry said. It is the first time the Japanese Ministry has confirmed Chinese and Russian warships going through the Strait, with its narrowest point only 18.7 kilometers.
A total of 10 vessels belonging to their navies sailed from the Sea of Japan to the Pacific Ocean, the Ministry said, adding Japan's Maritime Self-Defense Force found [that they were Chinese and Russian] destroyers and frigates...
The narrow [Tsugaru] strait is nonterritorial and used for international navigation so the passage of foreign ships [is legal]. But the Ministry said it is analyzing the two countries' intent.
China and Russia conducted a joint naval drill in the Sea of Japan this month and it is possible that some of the vessels were part of the exercise.”
Pete Comment
So, this time it is Chinese and Russian naval vessels that are legally conducting a Freedom of Navigation Operation (FONOP) even if this a worry for the US and Japanese navies.