Donald Trump has been ruffling diplomatic feathers since he was elected by casually talking to world leaders without first getting guidance from the State Department. He’s already angered close allies like Britain and India, but his latest phone call threatens to do far more damage.

That’s because of whom it was with: President Tsai Ing-wen of Taiwan.

This isn’t just some charming diplomatic faux pas by Trump — this was a blunder of potentially historic proportions. Trump’s call is believed to be the first between a US president-elect and a leader of Taiwan since diplomatic relations between the two countries were severed in 1979.

Trump’s transition team may have made matters even worse with its formal statement about the call, which referred to Tsai as the “President of Taiwan” — a title no American leader has used to refer to the head of Taiwan in decades. The word choice is certain to rankle China, which doesn’t recognize the legitimacy of Tsai or any of her predecessors:

President-elect Trump spoke with President Tsai Ing-wen of Taiwan, who offered her congratulations. During the discussion, they noted the close economic, political, and security ties exists between Taiwan and the United States. President-elect Trump also congratulated President Tsai on becoming President of Taiwan earlier this year.

Shortly after the transition team issued that statement, Trump took to Twitter and essentially doubled-down, once again referring to Tsai as the “President of Taiwan”:

The President of Taiwan CALLED ME today to wish me congratulations on winning the Presidency. Thank you! — Donald J. Trump (@realDonaldTrump) December 3, 2016

With this phone call, and the follow-up tweet, Trump is risking fundamentally upending decades of US policy toward Taiwan and enraging China, the world’s only other superpower.

China sees Taiwan not as an independent country, but rather as part of China

The dispute between China and Taiwan goes back to 1949 and the end of the Chinese Civil War, when the defeated Nationalists fled to the island of Taiwan, leaving the communists in power in mainland China. The two territories have been governed separately ever since, with both governments claiming to be the legitimate representative of "One China" — that is, China and Taiwan.

Most countries, including the US, only have formal diplomatic relations with mainland China and don't officially recognize the government in Taiwan — which is why Trump’s casual chat with the president of Taiwan, and his seemingly cavalier choice of words when describing Tsai, is so surprising, and so risky.

Trump, responding to the immediate groundswell of criticism after the phone call, fired back on Twitter:

Interesting how the U.S. sells Taiwan billions of dollars of military equipment but I should not accept a congratulatory call. — Donald J. Trump (@realDonaldTrump) December 3, 2016

His point is valid — the US does sell Taiwan billions of dollars of military equipment. But it betrays a clear lack of understanding of both US policy toward China and of how diplomacy works.

Under the 1979 Taiwan Relations Act, the US has pledged to “make available” to Taiwan various weapons and other military supplies “as may be necessary to enable Taiwan to maintain a sufficient self-defense capability.” It’s not a defense treaty, but rather a statement of policy that the US will help Taiwan maintain a defense against China.

Yet it also makes clear that the US does not formally recognize the government of Taiwan. It’s a policy known as “strategic ambiguity,” in which the US essentially tries to walk a very fine diplomatic line between supporting its friend Taiwan without upsetting the status quo or angering China.

China could choose not to take this too seriously

Of course, China could theoretically choose not to give too much weight to Trump’s actions, deciding that the president-elect doesn’t know what he’s doing — and isn’t signaling a fundamental shift in US policy. Diplomats from other countries privately say their governments aren’t currently taking Trump’s pronouncements seriously because they attribute them to ignorance, rather than a change of direction.

Indeed, a few hours after the incident occurred, China’s Foreign Minister Wang Yi, speaking at a conference, called the move a “petty act” by Taiwan and said it wouldn’t change the US’s longstanding “One China” policy.

But that doesn’t mean it won’t do damage.

Trump spent the campaign accusing China of manipulating its currency and taking advantage of American businesses, and threatened to take a harder line toward Beijing’s aggressive actions in the East and South China Seas. Trump may not realize it, but the US desperately needs Chinese diplomatic support at the United Nations and in reining in nuclear-armed North Korea. A decision by Beijing to dramatically ramp up selling off its vast holdings of American debt, meanwhile, could throw the US economy into recession.

That means that it doesn’t matter whether Trump meant to change US policy or not; Beijing may file away the call as it decides how to calibrate its relationship with the new administration — and whether to see Trump as a potentially ally or a potential adversary. Regardless of whether he "meant it" or not, China could take actions or make decisions based on those assumptions that could lead to insecurity and potentially even conflict.

“The Chinese leadership will see this as a highly provocative action, of historic proportions,” Evan Medeiros, former Asia director at the White House National Security Council, told the Financial Times.

“Regardless if it was deliberate or accidental, this phone call will fundamentally change China’s perceptions of Trump’s strategic intentions for the negative. With this kind of move, Trump is setting a foundation of enduring mistrust and strategic competition for US-China relations.”

On Saturday, China's Foreign Ministry reportedly said it had lodged "stern representations" with what it called the "relevant U.S. side," urging the careful handling of the Taiwan issue to avoid any unnecessary disturbances in ties.

But it now looks like it may actually have been intentional

Trump, somewhat defensively, tweeted shortly after that call that the Taiwanese president had called him — implying that he didn’t place the call intending to upend decades of US foreign policy, but had merely answered the phone when it rang. Vice President-elect Mike Pence, speaking on ABC’s “This Week” on Sunday, echoed this, stating, “He took the call, accepted her congratulations and good wishes and it was precisely that.”

But it now seems this may not be totally accurate.

Alex Huang, a spokesman for Tsai, told the Reuters news agency on Saturday that “Of course both sides agreed ahead of time before making contact.”

The Washington Post reports that the phone call was “an intentionally provocative move” by Trump that he and his advisers had planned months in advance. Citing unnamed people “involved in or briefed on the talks,” the Post says the call “was the product of months of quiet preparations and deliberations among Trump’s advisers about a new strategy for engagement with Taiwan that began even before he became the Republican presidential nominee.”

And it’s this confusion over whether Trump’s phone call was an intentional shift in US policy or just a gaffe that is the core problem here. If Trump wants to recalibrate US policy toward China and Taiwan once in office, that is well within his right to do as president, and indeed some foreign policy analysts would almost certainly support such a move.

But he must be absolutely clear that this is in fact what he is doing, so that both China and Taiwan, as well as all the other countries whose interests such a move could affect, know exactly where they stand. Otherwise, he risks introducing all sorts of chaos into the system, raising the risk of misunderstanding, overreaction, and potentially even conflict.

This is at least the third time that a Trump phone call has triggered controversy

Trump told British Prime Minister Theresa May, “If you travel to the US you should let me know” — as if sitting heads of state just pop into other countries unannounced.

He also managed to anger India by lavishing praise on Pakistani Prime Minister Nawaz Sharif and promising to visit Pakistan — something President Obama pointedly avoided doing during his two terms because of the two countries’ complicated relationship.

With this latest call, the president-elect hasn’t just caused a headache that Obama will have to deal with during his last weeks in office; he’s also given a worrying hint of the kind of diplomatic crises that may erupt once he moves into the White House.

Watch: How Trump thinks about foreign policy