First, the experts expect, China will try to persuade Donald Trump to stand down on Taiwan. It will marshal American business leaders to its side. Then it will threaten. And if that fails, it will sweep into action, pulling its ambassador from the U.S. and commanding military exercises in the already-dangerous waters of the Taiwan Strait.

Rocked by a U.S. president-elect who has dispatched with nearly four decades of tradition by speaking directly with the president of Taiwan in a phone call Friday, China will feel compelled to react, and perhaps strongly, say observers of the tense relations between Washington, Beijing and Taipei. China sees Taiwan as part of One China, its indivisible territory. The U.S. actively supports Taiwan, although it does not have formal diplomatic relations.

But first, Chinese leadership will have to determine if Mr. Trump's call with Taiwan's Tsai Ing-wen, his latest stunning and blithe foray into a potentially explosive foreign affairs issue, means anything at all.

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"It's just too premature to say he was signalling any intent to alter the U.S. approach and policy toward Taiwan. We don't know whether he really knew what he was doing," said Bonnie Glaser, senior adviser for Asia at the Center for Strategic and International Studies.

That question will occupy Beijing as well, as it formulates a response that is likely to give Mr. Trump space to change course before he takes residence in the White House – and, if that fails, strike hard.

China's first reply was conciliatory, with foreign minister Wang Yi saying he hoped relations with the U.S. would not be "interfered with or damaged."

"I don't think that it will change the One China policy maintained by the U.S. government for many years," he said in remarks reported by Chinese media Saturday.

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Mr. Wang laid any blame at the feet of Ms. Tsai for carrying out a "little trick." Mr. Trump seemed to do the same, in a Twitter message in which he emphasized that she called him, which suggested an attempt to defuse the situation.

The U.S. business community in China immediately struck a similar tone, urging Mr. Trump not to disturb the peace. "We expect the new administration to respect the status quo. American business operating in Asia needs certainty and stability," James Zimmerman, chairman of the American Chamber of Commerce in China, said in a Saturday statement.

In Washington and New York, too, China's diplomatic and business emissaries are likely to press Mr. Trump to leave Taiwan alone. "They will try to educate this incoming team about the dangers and the severe consequences of altering the U.S. approach to Taiwan," said Ms. Glaser.

But if Mr. Trump is serious in offering a more open approach to Taiwan, he will be seen in Beijing as threatening its sovereign territory, a breach of a red line – and a challenge to the very legitimacy of Communist rule in China – the response is likely to be furious. The ambassador could be recalled. Military exercises could be set in motion, with the U.S. forced to respond by dispatching aircraft carriers.

A fragile balance between China, Taiwan and the rest of the world could be upset.

Chinese president "Xi Jinping will feel compelled to react strongly. Not a good start for such an important relationship," said Phil Calvert, a retired Canadian diplomat with extensive experience in China and south-east Asia.

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"In recent years, China has tested each new American president early in his term in office," added Andrew Erickson, a founding member of the China Maritime Studies Institute at the U.S. Naval War College. Given the uncertainty over how Mr. Trump actually intends to approach Asia, "he is likely to face persistent Chinese probing from the very beginning," Prof. Erickson said. "With the world watching as almost never before, failure would come at a terrible cost."

China already distrusts Ms. Tsai, who it suspects of pro-independence leanings, and "will fear that this U.S. signalling, even if unintentional, could embolden Ms. Tsai to go further in this direction," Ms. Glaser said.

Mr. Trump counts a number of people with pro-Taiwan leanings in his inner circle, including most prominently senior policy adviser Peter Navarro, and on Twitter the president-elect suggested direct contact with Taiwan was only normal.

"Interesting how the U.S. sells Taiwan billions of dollars of military equipment but I should not accept a congratulatory call," Mr. Trump wrote. The Obama administration last year authorized a $1.83-billion (U.S.) sale of missile frigates and anti-aircraft systems to Taiwan, the first of its kind in four years.

Yet he is entering a fraught issue, one that has brought the two nations to the brink before. In 1995 and 1996, China fired missiles into Taiwan's territorial waters. The U.S. responded by sailing two aircraft carrier battle groups to the region.

Any reprise of those hostilities would be amplified by the tremendous gains China has since made in military might, which include over 1,200 short-range ballistic missiles in the Taiwan theatre, nearly 1,000 advanced fighter jets and hundreds of tactical nuclear weapons.

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"China has long been preparing to invade and murder Taiwan's new democracy. By the early 2020s China may have the capacity to successfully invade Taiwan for the first time since the 1950s," said Rick Fisher, senior fellow with the International Assessment and Strategy Center, a U.S. thinktank that works on security-related issues.

He argues that the only way to maintain peace is to encourage "a stronger Taiwan."

In Taiwan, many cheered the prospect of a U.S. president willing to defy China. The call with Ms. Tsai, "whether he's blundered into it or planned it, has a lot of potential for good," said Bruce Jacobs, a political specialist who lives in Taiwan. "It's time to confront the issue that Taiwan does not belong China and never has."

But the celebration is tinged with worry. "Taiwanese also realize that Trump is really an unpredictable bomb, and no one knows what will really happen," said June Lin, one of the student leaders during Taiwan's pro-democracy Sunflower Movement, and a policy fellow at the pro-Taiwan Formosan Association for Public Affairs in Washington, D.C.

In China, meanwhile, anger coursed through online conversations about Mr. Trump's convention-busting phone call. Most Chinese, like their government, see Taiwan as their territory, and many were surprised that Mr. Trump called Ms. Tsai "president."

"It's clear that he doesn't care about China's reaction at all. This is only his first bite. More will surely come," warned one commenter on China's Twitter-like Weibo social media.

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"This is very serious," wrote another. "Nothing like it has happened since 1979. One China is the bottom line, one that cannot be stepped on."

- With reporting by Yu Mei