TAIPEI, Taiwan — It's a 3.5 million-square kilometer stretch of ocean, speckled with some 200 coral atolls, some submerged or so tiny they hardly deserve to be called islands.

Welcome to the South China Sea, an obscure patch of global real estate that you're likely to hear more about in coming years.

Six Asian countries have long had competing — at times comical — claims to various islands here, sending token military forces to occupy barren rocks at great expense in the name of national pride.

What's new is China's muscle-flexing, which, if trends continue, could make the South China Sea one of Asia's most dangerous flash-points.

Fueling tensions in the sea are untapped oil and natural gas reserves, China's growing strategic interest in protecting sea lanes by which it gets some of its oil, and Beijing's desire to develop a "blue-water" navy capable of projecting power far beyond China's shores.

The U.S. is paying closer attention to the South China Sea, after China reportedly threatened U.S. energy firm ExxonMobil with retaliation if it continued oil exploration off Vietnam in waters China considers its own. And last year Chinese military vessels harassed U.S. surveillance ships in the sea.

Earlier this month, U.S. Defense Secretary Robert Gates made what's believed to be the highest-level public U.S. remarks to date on the issue.

"The South China Sea is an area of growing concern," he said at a security forum in Singapore. "This sea is not only vital to those directly bordering it, but to all nations with economic and security interests in Asia."

Gates repeated the U.S.' longstanding policy that it takes no position on conflicting sovereignty claims in the South China Sea.

But he said the U.S. believes "it is essential that stability, freedom of navigation, and free and unhindered economic development be maintained" and that "we object to any effort to intimidate U.S. corporations or those of any nation engaged in legitimate economic activity."

Here's a primer on the issue:

1) Why does America care?

The U.S. objects to any attempts to intimidate American energy companies operating in the South China Sea, which stretches from China all the way south to Indonesia. It also insists on the right of free navigation in international waters, defined, in accordance with customary international law, as any waters beyond 12 nautical miles from a nation's shoreline.

China says its sovereign territorial waters extend 200 miles from its shores, and makes a historical claim to almost all of the South China Sea, according to a backgrounder from the Heritage Foundation. China also says that any ship traversing the sea should first obtain Chinese permission. It has long complained about U.S. intelligence-gathering from spy-planes and spy-ships operating off its coastline.

2) Who else claims territory in the South China Sea?

Taiwan, Vietnam, the Philippines, Malaysia and Brunei also claim all or some of the South China Sea. Vietnam and China both claim the Paracels islands (known as the Xisha in Chinese), which China has controlled since a 1974 battle with Vietnam that left 18 dead. The other four countries as well as China and Vietnam also claim some or all of the Spratly Islands (known as the Nansha in Chinese) further south.

China's hold here is more tenuous; a skeleton force occupies nine speck-like islands, while Taiwan holds the largest island Itu Aba (or Taiping island, in Chinese), Vietnam holds 29 islands, the Philippines eight and Malaysia three, according to Michael Richardson, a visiting researcher at the Institute of Southeast Asian Studies, in a recent commentary. More than 70 Vietnamese sailors died in the latest military clash in the Spratlys, with China in 1988.

3) What's new about China's behavior?

China has built up its small military presence in the Spratlys. It angered Vietnam by issuing a unilateral fishing ban in the South China Sea, then boarding and seizing Vietnamese fishing boats who did not observe that ban.

Longer-term, China is building up a massive naval base on its southern island of Hainan from which it will be able to project power into the South China Sea. The base will house China's new nuclear-armed submarines, as well as its first aircraft carrier, expected to enter service by 2012, and many other warships.

Perhaps most significantly, China has recently begun to define its claim of sovereignty over the South China Sea as a "core interest," say analysts, using new language that puts the sea on par with Beijing's claims over Taiwan and Tibet. Veteran China watcher Willy Lam calls it part of China's "red-line diplomacy."

"These red lines define China's core interests," said Lam at a recent talk in Taipei. "Now, China is increasing its core interests. The latest development is that China also considers the South China Sea as its 'core interest' — it's asking the U.S. and other countries not to interfere with its 'core interests' in the South China Sea. It's drawing red lines around the entire sea."

Wendell Minnick, Asia bureau chief for Defense News, wrote in an email that Gates' remarks in Singapore were a "surprise."

"Clearly China's decision to include the South China Sea as a 'core interest' is something unnerving," said Minnick.

Southeast Asian nations are also increasingly worried, according to Arthur Ding, a Taiwan expert on military issues who also attended the Singapore conference. He said he's heard rising concerns from southeast Asian officials, especially those from Vietnam and the Philippines, about China's growing "assertiveness."

And he highlighted Chinese general Ma Xiaotian's mention of the South China Sea in a Q&A session in Singapore.

"The South China Sea had become so quiet, or at least not as much of an issue as the Taiwan Strait and the Korean peninsula," said Ding. "So this [Ma's remarks] really surprised me."

4) What are the most plausible conflict scenarios?

One worry is an incident at sea — say, a collision between a U.S. surveillance ship and a People's Liberation Army ship leading to loss of life — that could escalate due to miscalculation and lack of communication. One such incident took place in 2001 between a U.S. spy-plane and a Chinese fighter jet. The U.S. and Chinese militaries established a hotline in 2008, but China often simply refuses to pick up the phone out of pique, according to a recent Defense News report.

But the nations most likely to come to blows in the South China Sea may be China and Vietnam. Hanoi was incensed by Beijing's treatment of its fishing boats last year and lodged a formal protest. It continues to see the Paracels as its territory, illegally occupied by the Chinese. Anti-China nationalism runs strong in Vietnam and is easily inflamed. The two nations' militaries have twice skirmished in the South China Sea, in 1974 and 1988.

China's moves appear to have already started a regional arms race, analysts say. "Some southeast Asian nations are starting to beef up their armed forces to hedge growing Chinese assertiveness in the South China Sea," wrote Ian Storey, fellow of the Institute of Southeast Asian Studies in Singapore, in a recent commentary.

Defense News' Minnick said that Singapore, Malaysia and Vietnam had "gotten into the submarine game," and that "there are more concerns of an underwater collision than an accidental war" in the South China Sea.

"Many of the countries now deploying submarines are not familiar with underwater rules of right of way," Minnick said. "There are clear demarcations for direction and depth that are not being followed by some of the more inexperienced countries. And then there's Chinese submarines roaming around as well. So it's getting crowded underwater."

Meanwhile, one side effect of China's new claim may be to strengthen the U.S.' budding ties with Vietnam. "The U.S. is moving closer to Vietnam, and better military-to-military relations are expected to improve this year as China rattles the saber more," said Minnick.

5) Are there any efforts to resolve South China Sea disputes?

In 2002 the concerned nations signed a "code of conduct" agreement on the South China Sea. But the deal hasn't yet been fully implemented, largely due to China, analysts say. "China perceives the South China Sea as its territory, so it thinks 'Why do I have to implement the code of conduct?'" said Ding, the Taiwanese expert.

Last year, Vietnam and Malaysia submitted their formal claims to territory in the South China Sea. China immediately protested, rendering the claims invalid, a move that further ratcheted up ill will. The issue has quieted down in recent months, but the underlying territorial disagreements are far from being resolved.

"Somehow, ways must be found to prevent emotive nationalism and militarism from upsetting the uneasy status quo in the South China Sea," wrote the Institute of Southeast Asian Studies' Richardson.

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