Japan and South Korea, allies of the United States since World War II, are supposed to be part of an Asia-Pacific counter- balance to China's growing power and its expansive maritime and island claims in east Asia's seas.

Instead, an upwelling of nationalism as the region marked the anniversary last week of the end a war that devastated the region showed how fragile reconciliation is, even among nominal friends, decades after the fighting ended with Japan's surrender to US and Allied forces.

Today, it seems that ultra-nationalism among the leading local contestants in north-east Asia - China, Japan and South Korea - is gaining strength, raising the risk of armed conflict. The rhetoric was certainly strident as China demanded that Japan immediately and unconditionally release 14 activists who had sailed from Hong Kong and landed on the disputed Senkaku Islands in the East China Sea. Japan had no sooner deported the 14, when a group of Japanese nationalists landed on the Senkakus at the weekend despite an official ban, sparking anti-Japanese protests across China and adding fuel to the bitter dispute between Asia's two biggest economies over ownership of the islands - and the access to valuable fisheries and seabed oil and gas reserves that goes with them. Japan administers the uninhabited Senkakus in the teeth of opposition from China and Taiwan. Beijing says they belong to China and that its citizens have every right go there. It calls them the Diaoyu Islands.

The Global Times, part of a newspaper chain published by the ruling Chinese Communist Party, said that Japan now had to make a choice: ''Create conditions to reduce tensions over Daioyu, or head into a full confrontation with China. Whatever Japan's choice, China will respond accordingly.'' Renewed tension with China came just a few days after South Korea's President Lee Myung-bak inflamed another offshore dispute when he became the first South Korean leader to land on the rocky Dokdo islets in the sea between South Korea and Japan. The latter claims ownership of the islets, calling them Takeshima, even though they are garrisoned by South Korea.

In response, Japan recalled its ambassador to South Korea and cancelled a planned meeting of the two countries' finance ministers.

Both China and South Korea also used the anniversary of the ending of World War II in Asia to protest against the visit of two Japanese government ministers to a shrine commemorating Japanese who died in the fighting and its aftermath, including 14 convicted war criminals.

In separate statements, Seoul and Beijing called on Japan to atone fully for its brutal occupation of parts of China before and during the war, and for its 1910-1945 colonisation of Korea.

These issues have flared up before, but never in unison in a cascade of tension. What accounts for the surge of vitriol?

Politics has a lot to do with it. China, Japan and South Korea are all facing critical elections later this year. Being strong in defending offshore claims in the name of national pride and integrity is seen as a way to seek re-election or the appointment of candidates approved by outgoing incumbents.

If this analysis is correct, it holds out hope that calmer seas will prevail once the politicking season is over.

Yet jingoism is strongly entrenched in both China and South Korea where tapping into anti-Japanese sentiment is guaranteed to garner public support. Some close observers of Japan, where pacifism has prevailed since 1945, are also warning that virulent nationalism there is gaining a stronger foothold as the views of previously marginal right-wing politicians converge with those of younger lawmakers and voters concerned about Japan's economic future and its eclipse by China.

A Japan specialist at the Council on Foreign Relations in New York, Sheila Smith, has just returned from several weeks in Japan. She believes that the last serious clash over the Senkakus in 2010, after a Chinese fishing trawler rammed two Japan Coast Guard vessels, transformed thinking in Japan about the defence of its offshore islands. She says it stimulated for the first time a serious popular response that will make it more difficult to contain and manage tensions with China over the Senkakus in future.

Underlying the nationalism is an intensifying struggle for resource control and strategic advantage between China and Japan, the world's second and third largest economies respectively after the US. The Senkakus offer access to rich fisheries and potentially valuable seabed resources, including oil and natural gas. They are part of Japan's southernmost prefecture of Okinawa, a sparsely populated island chain. Key US bases there help Japan contain the expansion of Chinese naval power in the Pacific. The main island, Okinawa, is the hub of US airpower in the region. The Global Times last month called on the Chinese government to consider challenging Japan's rights to Okinawa prefecture on the grounds that it only became officially part of Japan in 1879 after centuries of paying tribute to Chinese emperors.

Just last month, Japan's Kyodo news agency reported from Washington a senior US State Department official had reaffirmed that if Japanese forces defending the Senkakus came under foreign attack, the US would be obliged to consider military intervention in support of its ally. Against this background, China is unlikely to try to risk taking the Senkakus by force in the immediate future. Instead, it will maintain its claim, built its strength and keep challenging Japan's control.

The writer is a visiting senior research fellow at the Institute of South-East Asian Studies in Singapore.