China's built-up, militarised island of Subi Reef is just above the center of the map above. With the US informing China beforehand, a US destroyer, USS Lassen, approached close to Subi on October 26, 2015. The South China Sea (see inset map above) stretches from Taiwan in the north to Borneo island and Singapore in the south, Vietnam in the west to the Philippines in the east. That sea's main trouble spots are the Paracel Islands and Spratly Islands (which includes Subi Reef).
The US has been the most active country to reject such a Chinese claim. The US is demonstrating the right of ships and aircraft to pass close to islands claimed by China by conducting freedom of navigation operations (FONOPs).
Submarines cannot show the flag unless running on the surface (at a major disadvantage) or sitting in port. But subs can precede, follow up and protect surface ships that are showing the flag.
When USS Lassen (DDG-82) embarked on her freedom of navigation operation (FONOP) on October 26, 2015 she “was accompanied by two [US] maritime surveillance aircraft, a P-8A Poseidon and a P-3 Orion. Pentagon officials leaked information about the FONOP before it took place as well as after it was completed, but refrained from speaking on the record about the mission.”
Lassen's FONOP took her within 12 nautical miles of Subi Reef. It is likely that Lassen was preceded by a US SSN from Submarine Squadron (SUBRON) 15, Guam, and possibly another SSN from that SUBRON followed her. These submarines, keeping watch with their extensive passive sonar and intercept systems, protected Lassen from surprises. Surprises would be mainly in the shape of Chinese surface warships and/or submarine activities.
China would anticipate the presence of US submarines for such politically and strategically important operation.
China for its part may have posted some Yuan class submarines along the previously advised route that Lassen took. I suspect Yuans were used because they would be China's best equipped subs to move into the Subi Reef area and wait for the Lassen on October 26. As the Yuan's have air independent propulsion (AIP) and other advances they would be better placed to discretely collect technical intelligence from the Lassen, other US ships and SSNs as these US ships/subs passed by..
DETAILS ON THE US SUBS AND USS LASSEN
Submarine Matters, on June 17, 2015, reported on Submarine Squadron (SUBRON) 15 which consists of 4 SSNs serviced by large shore installations and submarine tender USS Frank Cable all based in Guam. SUBRON 15 supports the activities of the Seventh Fleet headquartered in Yokosuka, Japan. At Yokosuka is Destroyer Squadron 15 which includes USS Lassen (DDG-82).
So although the Seventh Fleet is based in several far-flung places its powerful forces come together for key activities like the FONOP near Subi Reef.
It is unlikely that Australia or Japan will run their own FONOPs for the foreseeable future.
As indicated by Submarine Matters on September 24, 2015 China is building up several South China Sea islands into military bases very quickly. The islands can host air and naval bases and also long range intelligence sensors including radars. The islands it is building up most rapidly appear to be Mischief Reef, Fiery Cross Reef and Subi Reef.
Subi Reef, seen in three photos below, is being rapidly built up into an air and naval base - initially using sand dredging for landfill. The photos below, dated April 2015, June 2015 show rapid progress by August 2015 Subi. The reef-island has almost a completely enclosed harbour. Port facilities and a 3,000 meter airstrip will soon be built.
(Satellite image above courtesy Victor Robert Lee and Digital Globe via The Diplomat)
Subi Reef (above) 2 months later in August 8, 2015 (Satellite image courtesy CSIS/AMTI via Reuters)
China is effectively claiming that improving or building on islands is really a right of defence which (in China's mind) spreads out to a much broader territorial right around the islands. The air and naval bases that will eventually be built will also provide practical weapons to defend China's purported rights and arguments.
China's activities in the South China and East China Seas contribute to international tension. Tension that is drawing such countries as Japan, Australia, Vietnam, the Philippines, India and, of course, the US, closer together.