On December 31, Sankei Shimbun, one of Japan's five national newspapers, revealed that in February 2009, a Chinese nuclear sub crossed the so-called First Island Chain in waters near Taiwan. The term First Island Chain describes a maritime line which runs roughly between Korea and Japan's western coasts, through the Taiwan Strait along the northern half of the Philippines, and ending at the Indonesian island of Sumatra. To the US and its allies, any action by the Chinese navy beyond the First Island Chain symbolizes that Beijing is expanding its geographic limits or boundaries in terms of its military's operations.
The 2009 incident resembles one in November 2004, when a P-3Cs of the Japan Maritime Self-Defense Forces discovered a Han-class nuclear-powered submarine sneaking southwest of Ishigaki, an island belonging to the Okinawa Prefecture. That prompted the then-Japanese Prime Minister Junichiro Koizumi to bring into force a marine guard action directive to follow the sub. Together with the US, the Japanese subsequently monitored the Chinese sub making a cruise around Guam, a US key military hub, after which it sailed back to its mother port Jianggezhuang, a Chinese naval base 24 km east of Qingdao in the Yellow Sea. But while the 2004 incident led to considerable media coverage, the 2009 break through the First Island Chain was not detected by the Taiwanese.
Although the US has responded to the rapid development of the Chinese submarine fleet by deploying the ocean surveillance ships USNS Victorious and USNS Impeccable, which monitor the seas with low-frequency sonar arrays and The Japanese self-defense forces have also strengthened surveillance with their 100 P-3Cs, both the 2004 and 2009 incidents make it obvious that US and Japan have been gradually losing the ability to fully monitor Chinese subs' movements.