This is the fourth installment of a series.

“It changed its direction to west,” an official of the Maritime Self-Defense Force said.

On the morning of Jan. 11, a Shang-class nuclear-powered attack submarine of the Chinese Navy changed course after initially moving north in waters east of Taishojima island, which is part of the Senkaku Islands in Okinawa Prefecture. The change sent a wave of alarm among the crew of the MSDF destroyer Onami, which detected the submarine with sonar.

The submarine was heading to the contiguous zone around the Senkaku Islands. If it had moved further west and entered Japan’s territorial waters, a maritime security operation would have been ordered to allow the Self-Defense Forces to exercise police authority.

Hirotaka Okumura, commander of the MSDF destroyer, instructed his crew to “not lose sight of it.”

The submarine cruised into the high seas after entering the contiguous zone. The Onami made its engine fully operational, pursued the vessel and secured photographic evidence.

The Japanese government strongly protested China’s provocation, describing it as “a serious escalation of the situation.”

The Chinese military strategy, called anti-access and area denial (A2/AD), consists of two stages — preventing the U.S. military from operating in the area called the “Second Island Chain,” which stretches from the Izu Islands to Guam, and preventing the U.S. military from entering the area called the “First Island Chain,” which stretches from the Nansei Islands to the Philippines.

The Chinese military has strengthened its activity in an area near Okinawa Prefecture, as the area is located at a strategic point in the First Island Chain.

China’s first aircraft carrier Liaoning was confirmed to have advanced into the Pacific Ocean for the first time by sailing in waters between Okinawa Island and Miyakojima island in December 2016. Bombers and fighter jets also fly over the area frequently.

A senior Defense Ministry official said: “China has reinforced its control of the South China Sea through landfill work on reefs and rocks. It will next likely increase pressure to turn the East China Sea into ‘China’s sea.’”

The Japanese government has also taken action. In the “National Defense Program Guidelines,” a basic defense policy devised in 2013, the government worked out a plan to develop a “Dynamic Joint Defense Force,” in which Ground, Maritime and Air Self-Defense Forces operations will be more closely integrated, with the aim of strengthening defense of the Nansei Islands.

It is also proceeding with deploying SDF units in Okinawa Prefecture. A GSDF Coast Observation Unit was deployed on Yonagunijima island, the nation’s westernmost point, in March 2016. The deployment of a security unit in Miyakojima island is also scheduled for March 2019.

On March 27, an Amphibious Rapid Deployment Brigade of the GSDF was newly formed at Ainoura Camp in Sasebo, Nagasaki Prefecture, as a measure to protect remote islands. In the event of a foreign incursion on an island, including the Senkakus, the unit will play a key role in regaining control with the use of amphibious vehicles and Osprey transport aircraft slated to be introduced this autumn.

However, with a contingency on the Senkaku Islands in mind, a major task is dealing with a possible armed attack as well as a “gray zone situation,” which refers to a vaguely invasive action that heightens the situation above peacetime levels but is not recognized as an armed attack by a foreign country.

A likely scenario includes the occupation of an island by armed fishermen and others; a response could be delayed, as the command of the first response will be left to the Japan Coast Guard and police force.

In the process of considering the security-related legislation, discussions were held on the idea of developing a legal framework to authorize the SDF to defend the nation’s territories even in peacetime. But the territoriality of relevant organizations hampered the idea from becoming reality.

On March 21, China announced a restructuring plan to shift control of its coast guard to the command of the Central Military Commission. As government vessels of the coast guard have repeatedly entered Japan’s territorial waters near the Senkaku Islands, it can be assumed that the Chinese military and coast guard would coordinate an advance to the islands.

Given the situation, the Japanese government intends to deal with it through measures such as a “maritime and aerial liaison mechanism” to prevent accidental clashes between the SDF and the Chinese military — on which the two countries are expected to formally agree soon.

The government also plans to strengthen the nation’s defense capability on remote islands further under the new National Defense Program Guidelines, to be devised at the end of this year, and the next Medium Term Defense Program.

Kiyofumi Iwata, former GSDF chief of staff, said: “Work on measures to defend the Nansei Islands has just begun. It is necessary to further increase the command communication capability, transportation ability and amount of ammunition, among others.”