Donald Trump’s policy of calling every world leader just took a dramatic turn. His day’s roster of congratulatory conversations included a phone call with Taiwanese president Tsai Ing-wen—demolishing the long-held US “One China” policy, in place since Richard Nixon was in office.

The Trump transition team wrote the following in a description of the call shared with the press:

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.

This simple call between Trump and Tsai—the leader of Taiwan’s broadly pro-independence party, elected president in January—is sure to infuriate China. Taiwan is an independently run democracy, but China views it a renegade province, and part of its territory. The US “One China” policy states unequivocally that Taiwan and China are part of the same country.

Since 1979, there has been a strict ban on direct communications between the US and Taiwan—let alone heads of state or presidents-elect. The US maintains a careful balancing act, selling weapons to Taiwan and serving as the de facto guarantor of its safety, while avoiding provocations against Beijing.

The call was set up by a Trump staffer friendly to Taiwan, according to the Taipei Times. To complicate matters further, Trump’s real estate company has expressed interest in building luxury hotels in the Taiwanese city of Taoyuan, according to its mayor (link in Chinese).

It would be difficult to overstate how serious an affront this is to China. Whenever any country deals directly with Taiwan—whether it is the US selling weapons or Cambodia setting up direct flights to Taipei—China lashes out, reminding the world of its stance that Taiwan and China are one and the same.

The delicate relationship between Taiwan and China is one of the world’s biggest military flashpoints. It is explicit Chinese policy that any declaration of independence from Taiwan would trigger an automatic invasion, and China is believed to, at all times, have thousands of missiles pointed at its small island neighbor.

In another freelance foreign policy move, Trump had a bizarre phone call with Pakistani prime minister Nawaz Sharif yesterday (Dec. 1), where he offered to help mediate the country’s decades-old dispute with India, upending a delicate relationship in a notoriously unstable region. Adding China to the mix with India, Trump appears to have angered one-third of the world’s population in just 48 hours.