A simmering territorial and maritime dispute in Asia erupted at the weekend as Washington warned Beijing it would back Japan militarily in any confrontation arising from China's latest unilateral assertion of its claims.
The Obama administration weighed in after China moved to in effect "rope off" the seas and skies around the disputed Japanese-administered Senkaku islands in the east China Sea.
In a tough statement reflecting the surprise and alarm felt in Washington and Tokyo at China's perceived sudden escalation of the dispute, Chuck Hagel, defense secretary, said the US was "deeply concerned" at the development, in which China appears to be trying to control who can enter and leave the area.
The imposition of the zone was a "destabilizing attempt to alter the status quo in the region", Hagel said. "This unilateral action increases the risk of misunderstanding and miscalculations ... We are in close consultation with our allies and partners in the region, including Japan. We remain steadfast in our commitments."
Hagel reminded Beijing that the remote Senkaku islands, known as Diaoyu in China, are covered by the 1952 US-Japan security treaty, under which the US is committed to fighting alongside Japan to repel any "common danger".
Washington's swift intervention showed just how easily a little local difficulty in the volatile east Asian region could potentially trigger a superpower clash. The Senkaku stand-off is but one of several similar disputes pitting a more assertive China against its less powerful neighbors.
Vietnam, the Philippines, Brunei, Malaysia and Taiwan (all US allies) are – like Japan – enmeshed in arguments with Beijing over relatively obscure but potentially strategic bits of maritime real estate.
These numerous flashpoints have led the Japanese government to describe the regional security environment as "increasingly severe". In response, Tokyo has been busily building up mutual defence and security ties across south-east Asia, and with Australia and India, as a hedge against Beijing.
For its part, China has sought to enlist Laos, Cambodia, North Korea and Myanmar as de facto buffer states while projecting itself as a rival to the US as a "blue-water" Pacific power.
Japan has denounced the zone set up by China on Saturday as "totally unacceptable", and indicated that aircraft from its self-defense force would ignore Beijing's attempt to oblige airplanes to obtain its permission before entering.
Fumio Kishida, foreign minister, warned that China's action "could well lead to an unforeseen situation". He called the development "very dangerous". Hagel said that US forces in the Pacific theater, including those based in Japan and South Korea, would also ignore China's strictures.
The Chinese government-run Xinhua news agency published map co-ordinates for what it called the "East China sea air defense identification zone" covering most of the sea and the skies over the islands. It said China's armed forces would take "defensive emergency measures" against aircraft that failed to identify themselves properly or follow its radio instructions.
Xinhua claimed the "air zone could contribute to regional peace and security by curbing the increasing rampancy of Japan's right-wing forces, as well as the continuous and dangerous provocations of Japanese politicians, which even Washington should be vigilant against".
The statement was an apparent reference to Shinzo Abe, Japan's conservative prime minister, who was elected last December on a platform of standing up for Japan's rights. Abe, who says he is intent on making a "proactive contribution to peace", has been denounced in China and South Korea as a reckless nationalist and historical revisionist.
Although China and Japan share two-way trade worth $250 billion (£150 billion) a year and maintain many other bilateral links, Abe and the Chinese president, Xi Jinping, have yet to hold a summit meeting, and other high-level contacts remain frozen.
The dispute over the Senkakus, situated south-west of the Japanese mainland, dates back to 1971 when China claimed sovereignty. Up to that point, the islands had been under unchallenged Japanese control since the 19th century, although Taiwan also has a claim.The row escalated last year when Tokyo effectively nationalized some of the chain, purchasing two islands from private owners. The takeover provoked street protests in China and furious official condemnation. Since then there have been almost daily incursions by Chinese fishing boats and other non-military vessels.
Although tiny in size, the Senkakus are thought to sit on top of valuable energy deposits. But analysts say the outcome has broad implications for regional security, given that an ever more assertive China is prosecuting similar disputes with several other neighbors.
Akio Takahara, professor of international relations and law at Tokyo university, said: "[Senkaku] must be viewed as an international issue, not just a bilateral issue... and it is very, very dangerous. They [China] must stop the provocations. If Japan did buckle, it would send a very bad message."
In a foretaste of what could happen in the weeks ahead, China sent an air patrol into the zone shortly after the official announcement. Later on Saturday afternoon, Japan scrambled fighter jets after two Chinese reconnaissance planes appeared over the East China sea.
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