Soon after dawn two weeks ago the captain of the Japanese destroyer Atago was on the bridge of his ship cruising within territorial waters off southwestern Japan when he saw something in the water about a kilometre away.
"Isn't that a periscope?" he asked.
Crew detected the target with the ship's state-of-the-art sonar. They then "pinged" what they took to be a submarine with their targeting sonar.
Under the rules of the sea the submarine should then have surfaced and displayed its national flag or faced being attacked by the Japanese warship with depth charges or torpedoes.
But, as the submarine captain was doubtless well aware, Japanese armed forces are heavily constrained by the country's pacifist constitution.
The submarine sped off, immune from attack, and revived a sharp debate in Japan about the constraints on the military at the time of a substantial arms race in Asia.
The Japanese believe, after talks with United States allies, that the submarine was Chinese and part of a now large naval force Beijing has been building and deploying in recent years to back its claim to be a regional power.
Indeed, Beijing has put special emphasis on creating a large and sophisticated submarine fleet as the cheapest and most effective way of projecting power well beyond China's coastal waters.
China's submarine fleet is now one of the world's largest with nearly 85 vessels. More than that, old and unreliable boats mostly acquired from the old Soviet Union are being rapidly replaced by modern submarines armed with highly sophisticated anti-ship missiles and radar-dodging cruise missiles able to attack land targets.
Beijing is even building at least five ballistic missile submarines, each carrying 12 intercontinental missiles and each missile having three nuclear warheads.
Meanwhile Japan has 16 submarines and no plans to build more and the American Pacific Fleet has 35 submarines, the world's most modern.
Beijing's emphasis on naval construction in recent years raises many eyebrows among military planners in Asian and Pacific Ocean nations because the intentions behind this huge investment in military power are so unclear.
It was again a central topic at the Maritime Security Challenge '08 conference of experts on Asian naval issues organized by Canada's Maritime Forces Pacific at Victoria last week.
At first the supposition was that Beijing's naval expansion aimed at backing its threats to invade and capture the independent nation state of Taiwan.
But the development of China's navy, both of surface warships and submarines, has now gone well beyond what is necessary to invade Taiwan and deter the island's main ally, the U.S., from rushing to its aid.
The vulnerability of the U.S. navy to attack from Chinese submarines was demonstrated with stark clarity in October 2006. A Song-class diesel-electric submarine shadowed a U.S. battle group led by the aircraft carrier Kitty Hawk and was only detected when it surfaced close to the carrier to reveal its presence.
That incident undermined the confidence with which U.S. strategists always talk about the superiority of their naval technology.
So does the coming into operation earlier this year of a massive submarine base hollowed out of the cliffs of China's Hainan island in the South China Sea.
The base at Sanya can house about 20 submarines and the entrance is so large they can leave and return submerged so as to be undetectable by satellites.
The base comes into operation at a time of increased tension between Beijing and other countries, especially Vietnam, around the South China Sea. Beijing claims most of the sea is Chinese territorial waters, and therefore it owns the resources underneath it.
Beijing's determination to project power goes well beyond its claims to the South China Sea, however.
China's economic well-being and therefore its internal security is now wholly dependent on sea-borne trade. And most of that trade, especially vital imports of oil, come across the Indian Ocean and through the South China Sea.
This has brought China's navy face-to-face with regional rival India, which is rapidly developing its own substantial maritime force into one that can make its presence felt throughout the Indian Ocean and even into Southeast Asia.