The South China Sea dispute is heating up again with Vietnam and the Philippines claiming parts of Nansha and Xisha islands as their territory and accusing China of aggression in the resource-rich waters off the islands.

Two incidents in the last century, however, establish China's sovereignty over Xisha and Nansha islands.

In 1909, Zhang Renjun, then governor of Guangdong and Guangxi evicted a Japanese merchant who was illegally occupying part of the Dongsha Islands. After that, he realized the necessity of defending the other islands in the South China Sea and asked navy commander Li Zhun to patrol the waters off the Xisha Islands with three warships.

Li raised the dragon flag of the Qing Dynasty (1644-1911) on the islands, emphasizing they were part of China's territory. Later, the Chinese navy drew charts and made a plan to exploit the islands.

This fact is accepted internationally. The British Navy's China Sea Pilot, published in 1938, says China lists the islands as its territory.

Then on Sept 29, 1932, China's minister to France wrote to the French foreign ministry saying that France had not protested against China's claim over Xisha Islands in 1909.

International law stipulates that effective occupation is the precondition of owning any island or group of islands. Chinese fishermen from Hainan Island had long been residing on Xisha Islands, and the then Qing navy's action reaffirms the islands belong to China.

Another key historical moment is the return of Taiwan after World War II. The 1943 Cairo Declaration, co-signed by China, the United States and Britain, says Taiwan and its subsidiary islands occupied by Japan should be returned to China. In the 1945 Potsdam Proclamation, the three countries reiterated the Cairo Declaration, and China took back Taiwan, as well as Xisha and Nansha islands.

In autumn 1946, China sent a fleet to Xisha and Nansha islands. On Nov 24, two warships reached the main island of the Xisha Islands and the Chinese built a monument there. On Dec 12, two other Chinese warships reached the main island of the Nansha Islands and named it Taiping Island.

According to international law, a country can claim sovereignty over an archipelago if it occupies the main island. Therefore, China's sovereignty over Nansha and Xisha islands is not contestable.

Vietnam intruded upon the Nansha Islands in 1956, when the then South Vietnam government sent marine troops to one of the largest reefs of the Nansha Islands. The South Vietnam government declared Nansha Islands as part of its Phuoc Tuy province in 1973, and granted some foreign-funded companies "permission" to explore the waters for oil.

After reunification in 1975, Vietnam took over the reefs previously controlled by South Vietnam and continued to intrude upon other reefs - at least 29 by now. Besides stationing troops and erecting military bases, Vietnam has also built airports and meteorological stations, and set up other facilities on some large reefs.

Vietnam bases its claim over the Nansha Islands mainly on its so-called historical occupation, control and exploration of the islands. But if that is the case, Vietnam would be contradicting itself because it acknowledged China's sovereignty over the islands from the 1950s to the 1970s.

In 1956, Ung Van Khiem, the vice-foreign minister of the Democratic Republic of Vietnam (DRV or North Vietnam), acknowledged that historically the Nansha Islands were a part of Chinese territory when he met with Li Zhimin, China's charge d'affaires in Vietnam. On the same occasion, another high-level Vietnamese official even said that according to Vietnamese sources, China's claim over the islands went back to the Song Dynasty (960-1279).

In 1958, shortly after China issued a statement on its territorial waters, including the Nansha and Xisha islands, Pham Van Dong, then premier of DRV, said Vietnam respected China's sovereignty statement on its territorial waters. Thus Vietnam has long recognized China's sovereignty over the islands in the South China Sea.

Some Vietnamese maps published in the 1960s and the 1970s even mark the Nansha Islands as part of Chinese territory. Moreover, a Vietnamese geography textbook published in 1974 depicted the islands in the South China Sea, including Nansha and Xisha islands, as an arch and compared it to a "great wall" at sea safeguarding the Chinese mainland.

As for the Philippines, its interest in the Nansha Islands began shortly after it gained independence in 1946. But it did not clear its stance until 1971 when studies showed the waters could be a storehouse of oil. In 1978, then president Ferdinand Marcos issued an ordinance claiming sovereignty over the Nansha Islands (spread over an area of about 64,976 square nautical miles), and named them the Kalayann Island Group.

The Philippines bases its claim over the islands on three premises. First, it says the islands are the lifeline of its national security and economy. Second, the Philippines is geographically closest to the islands. And third, it claims that before the Philippines laid claim, the islands did not belong to any country. But the reasons cannot justify the Philippines' sovereignty over the islands.

No country can claim sovereignty over any land or water body which belongs to another country by citing economic and security concerns. No matter how desperate the Philippines is to control the resource-rich waters, it cannot jeopardize China's interests by claiming the islands.

Also, geographical closeness does not necessarily mean the islands belong to the Philippines. Take the Christmas Island for example. Although it is hundreds of nautical miles away from the Australian mainland and less than 200 nautical miles from the Indonesian island of Java, it is still part of Australian territory. So the Philippines cannot claim the Nansha Islands based on geographical closeness.

More importantly, the Nansha Islands were not res nullius before the Philippines' claims. China's sovereignty over the islands was established long ago. And although China's sovereignty over the islands weakened in modern times, it reaffirmed it after the end of World War II, enduring to defend them against intrusions by Vietnam, the Philippines and other countries.

On more than one occasion, Vietnam has acknowledged China's sovereignty over the islands in the South China Sea, and the Philippines cannot justify its claim. Hence, both countries should acknowledge China's sovereignty and stop distorting history.

The author is a professor at the Center for Southeast Asia Studies, Xiamen University, Fujian province.