Week by week, China solidifies its hold over the South China Sea, resisting all attempts to rein it in. It ignored a damning ruling last month by an international tribunal in The Hague, dismissing its claims to a sweeping arc hundreds of miles south and east of Hainan, its most southerly province.

Beijing has reclaimed nearly four million sq metres (43 million sq ft) of land from one section of the sea, according the US-based Center for Strategic Studies. A manmade island now hosts a lighthouse, port and radar towers. It has landed military aircraft on a newly built 3,000m-runway on Fiery Cross Reef.

All this will test Tomomi Inada, Japan's new defence minister, appointed this week. Tokyo has no direct claim to the seas but a clear interest in a region through which €4.5 trillion in trade passes every year, much of it Japanese. Increasingly, Japan supplies military equipment and training to Vietnam and the Philippines, the two nations most likely to clash with China.

Beijing’s claims to most of the South China Sea, and to Japan-held islands on the East China Sea, are commonly denounced as a product of its hyper-nationalism and restless search for resources. There may be another plausible reason for the growing brinkmanship: shrinking oceans.

Economic zone

Although the two nations have roughly the same amount of coastline (China’s 30,000km v Japan’s 29,000km), Japan has a total exclusive economic zone (EEZ) of 4.5 million sq km in the high seas, five times more than its bigger, more populous neighbour. And Japan’s maritime domain has vastly expanded in the last three decades.

Until recently, the high seas were commonly owned. But since the United Nations Convention on the Law of the Sea was ratified more than 30 years ago, about 160 countries have carved up the oceans into EEZs, giving them special rights to up to 350 nautical miles (650km) beyond their territorial waters.

Nations such as Britain, France and Japan, with residual territories from far-flung colonial empires have arguably done better out of this arrangement than China, which ranks between the Maldives and Somalia as a territorial maritime power. Resentment at this colonial history helps to explain the vociferousness of Chinese claims.

Tokyo takes these EEZs seriously, as its jurisdiction of a string of islands extending into the Pacific shows. At the farthest reaches is Okinotorishima, (literally “remote bird island”), almost 2,000km from the Japanese capital, roughly the same distance from London to Reykjavik.

Essentially two coral reefs, the territory shrinks at high tide so that “one is about the size of a double bed and the other a small room,” according to Gavan McCormack of Australian National University. Since 1987, he says Tokyo has invested $600 million (€540 million) in an attempt to shore up the reef and stop it from disappearing under the rising seas.

The rewards are not hard to understand: an EEZ attached to a fixed point on the dubiously defined “island” would give Tokyo a theoretical maximum of 1.3 million sq km, three and a half times the total land area of Japan.

Human habitation

China says this is not an island but a rock and cannot support human habitation. The argument mirrors criticism that China’s reclaimed land in the South China Sea should be categorised as “low-tide elevations” and “rocks”. All sides may settle such disputes, says

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, by relying on that old saw: possession is nine-tenths of the law.

Japan's purchase in 2012 of three islands, which it calls the Senkakus (and China calls the Diaoyus) has increased regional tensions. Washington plans to concentrate 60 per cent of its navy in the Pacific by 2020, increasing the strategic importance of the islets, which are administratively part of the Okinawa chain, host to the heaviest concentration of US military forces in Japan.

Says McCormack: “From the Chinese viewpoint the Okinawan islands resemble nothing so much as a giant maritime Great Wall . . . potentially blocking naval access to the Pacific Ocean.”

The risk of conflict in an area with all these overlapping claims can hardly be overstated. Since the defeat of Japan in the second World War, America has been the only nation rich enough to project military power across the region – until now. China is increasingly inclined to challenge that US dominance.

Inada's appointment as defence minister may add a little more fuel to this combustible mix. She has long been associated with the hard right of the ruling Liberal Democratic Party (LDP) and supports a larger military role for Japan. She has also questioned the legal basis of the post-war Tokyo Tribunal, which condemned atrocities committed by Japanese soldiers during the war. The biggest victim of these atrocities was China.