US president Barack Obama recently declared that a 1960 defence treaty obligates his country to defend Japan if disputed islands in the East China Sea are attacked.
Called Diaoyu in Chinese or Senkaku in Japanese, the islands are "under Japan's administration", Mr Obama noted. It is their ownership that China, the potential attacker, disputes. Making up a total of under 7 sq km in area, the five uninhabited islets and three barren rocks lie slightly on Japan's side of the median in the East China Sea, about 120 nautical miles east of Fuzhou, China, 90 nautical miles north-east of Taiwan and 90 nautical miles from the Japanese Ryukyu islands.
Their value lies below, in "the most prolific oil and gas reservoirs in the world, possibly comparing favourably with the Persian Gulf area", according to a 1968 US Naval Oceanographic Office study. This study sparked the dispute, as whoever owns the islands owns the oil and gas.
Japan has been administering the islands since 1972, maintaining a lighthouse and patrolling the waters. China insists that Japan occupies the islands illegally.
Neither side has been willing to submit to international adjudication or arbitration. Both sides, having used the islands to stoke nationalist feelings, fear their citizens will not accept an adverse decision, which would undermine the ruling party's legitimacy.
For China, additionally, surrendering its claim to the islands would reflect badly on its claim to Taiwan as well. Now throw the United States into the mix and the islands could well become a flashpoint in North-east Asia.
For these reasons and because the world's three largest economies are involved, the dynamics in this dispute differ significantly from the one in the South China Sea. Here, two big sets of concerns - geophysical and treaty - matter the most.
The East China Sea is a semi-enclosed body of water, just 345 nautical miles across at its widest point between China and Japan. A median line would give each state less than the 200 nautical miles exclusive economic zone (EEZ) permissible under the United Nations Convention on the Law of the Sea (Unclos). A coastal state has sovereign economic rights over the marine resources within its EEZ.
Associate Professor M. Taylor Fravel, a political scientist at the Massachusetts Institute of Technology, tells The Straits Times, however, that - although often used by claimant states - the median line concept is not found in the text of Unclos. In fact, Unclos has exacerbated matters as it also allows a coastal state to claim, alternatively, an EEZ of up to 350 nautical miles from its shores, depending on where its continental shelf ends.
As such, China is claiming that its continental shelf ends at a deep trench in the seabed called the Okinawa Trough, which lies up to 77 nautical miles beyond the 200-nautical mile boundary (but does not exceed 350 nautical miles from its shores). The trough is 900 km long, 230 km at its widest and 2,700 metres at its deepest. It extends from the north of Taiwan to the south-west of Kyushu, Japan.
Shanghai Jiao Tong University law professor Julia Xue Guifang tells The Straits Times that the trough forms a natural boundary between China and Japan. China has offered ample geophysical evidence that there is indeed "a continuous, evenly extended landmass, a natural prolongation from its shores that terminates at the trough". This means the continental shelves of China and Japan are not connected.
Moreover, the trough was seen as a natural boundary between China and Japan even in ancient times. Professor Xue notes that 16th-century Chinese navigation records tell of sailors going to the Liuqiu Kingdom (Okinawa) to collect tribute. These ancient mariners noticed changes in the colour of the waters when sailing over the trough, when they would sacrifice live pigs and sheep to the sea god. This haishenji (ritual to the sea god) was also called guogouji (the trough-crossing ritual).
However, Japan counters that the whole sea is made up of the East China Sea Basin on the west and the Okinawa Trough on the east, whose total cross section is a distorted W, with the W's middle peak being an uplifted belt, upon which the islands sit.
This uplifted belt, Japan argues, is a natural prolongation of not just China's mainland but also the Ryukyu islands 90 nautical miles away, so the disputed islands are really a part of Japan's Ryukyu islands.
Under Unclos, when continental shelves of two coastal states are continuous, as Japan argues, no feature like the trough may serve as natural marker to separate their EEZs. If so, the median is the fairest and the islands would fall within Japan's side.
Yet, under Unclos, Prof Fravel notes: "China as a larger country would probably be entitled to a bit more than half of the EEZ."