CHUMPUP ALREADY CAUSING PROBLEMS

A call between U.S. President-elect Donald Trump and Taiwanese President Tsai Ing-wen is shaking delicate relations between Beijing and Taipei and threatening to place the U.S. in the middle of a relationship it has been careful not to upset.

China said it had lodged a formal complaint with the U.S., and Taiwan urged China to stay calm a day after Mr. Trump broke diplomatic protocol by speaking by phone with Taiwan’s leader.

“We must point out, there’s is only one China in the world and Taiwan is an inseparable part of Chinese territory,” read a statement posted on the Chinese Foreign Ministry’s website Saturday afternoon.

Taiwan split from mainland China in 1949, when Chiang Kai-shek’s Nationalists fled there to escape Mao Zedong’s Communist forces after years of civil war. Beijing considers the self-ruled, democratic island a breakaway province.

In the roughly 10-minute phone call Friday—believed to be the first time a U.S. president or president-elect has spoken with the leader of Taiwan since Washington established diplomatic ties with Beijing in 1979—Mr. Trump and Ms. Tsai discussed economic and security ties and congratulated each other on their respective election victories, according to statements from both sides.

“It’s normal for President Tsai to congratulate the U.S. president-elect and emphasize the maintenance of friendly relations in the future,” Taiwan’s Mainland Affairs Council said in a statement sent to reporters Saturday. “The mainland Chinese side should view this in a calm manner.”

The council’s mainland Chinese counterpart, the State Council Taiwan Affairs Office, said the call “can’t change Taiwan’s status as a part of China.”

Even if Mr. Trump isn’t yet president, the call strikes a raw nerve in Beijing, where anxieties are already high over Taiwan’s political direction and Washington’s role in that.

Ms. Tsai leads a party that supports Taiwan’s independence from China. Ever since her election, Beijing has repeatedly urged her to commit to maintaining political arrangements it reached with her predecessors. Though Ms. Tsai has said she would keep to the status quo, she has declined to explicitly endorse terms Beijing sees as crucial to the relationship.

Ensuring stable relations with Taiwan is crucial for Chinese President Xi Jinping, who has projected himself as a strong leader able to defend the country’s interests as he seeks a renewed mandate for a second five-year term.

From Beijing’s perspective, the Trump team “is signaling support for a leader seen as pro-independence, and that could embolden her. That’s very dangerous,” said Bonnie Glaser, an Asian affairs specialist at the Center for Strategic and International Studies in Washington.

By reaching out to Ms. Tsai, Mr. Trump’s team is reinforcing long-held concerns by Beijing that Washington is encouraging Taiwan to resist Chinese entreaties to reunify. Those worries had subsided for much of the past decade, as Ms. Tsai’s predecessor, Ma Ying-jeou, pushed a broad engagement with Beijing, and Taiwan fell on the list of irritants between Beijing and Washington. That changed with the election of Ms. Tsai earlier this year.

Taiwan was the first topic raised by Mr. Xi in a meeting with President Barack Obama on the sidelines of an economic summit in Hangzhou in September, with the Chinese leader speaking at length about China’s concerns, people briefed on the meeting said.

Sean King, an Asia specialist and senior vice president at consulting firm Park Strategies, called the phone call a “major diplomatic coup” for Ms. Tsai that will make Taiwan feel more secure and relevant. He played down the risks to Taiwan of a closer relationship with the U.S. “The status quo is fine. Beijing will only attack Taiwan if Taipei declares independence,” he said.

Others were less sanguine. Susan Shirk, a former U.S. diplomat who chairs the 21st Century China Program at the University of California, San Diego, described the balance across the Taiwan Strait as “very delicate” and said it was “highly irresponsible” for Mr. Trump to take actions that might upset it.

Earlier Saturday, Chinese Foreign Minister Wang Yi dismissed the call in comments to local media as “a petty trick” on the part of Ms. Tsai, that wouldn’t affect U.S. support for the “One China” principle.

New Republican administrations have previously put Beijing on the back foot by signaling stronger support for Taiwan. The administrations of George W. Bush agreed to more robust sales of weapons and defense equipment to the government of then-Taiwan President Chen Shui-bian, like Ms. Tsai, a member of the pro-independence Democratic Progressive Party.

Tensions with China soared under Mr. Chen. Mr. Bush, dealing with the wars in Iraq and Afghanistan, ended up upbraiding Mr. Chen for adding to instability in East Asia