Pelosi’s military aircraft arrived at Songshan Airport at 10:43pm local time, Taiwan’s Central News Agency reported.

China, which regards Taiwan as part of its territory, vowed an unspecified military response ahead of Pelosi’s visit that risks sparking a crisis between the world’s biggest economies. President Xi Jinping told President Joe Biden last week he would “resolutely safeguard China’s national sovereignty and territorial integrity” and that “whoever plays with fire will get burned.”

Traders braced for bad news ahead of the visit, with stocks sliding and haven assets such as the yen and Treasuries climbing. While there are few signs China is planning a full-scale invasion of Taiwan, Beijing has responded to past visits by foreign officials with large sorties into Taiwan’s air defense identification zone or across the median line that divides the strait. 

Taiwan faced cyberattacks ahead of Pelosi’s arrival, with the presidential office saying it suffered a 20-minute barrage in the early evening hours that was 200 times worse than usual. The website of the Foreign Ministry also appeared to face periodic disruptions.

Taiwan’s Defense Ministry said in a statement Tuesday that the island’s military was prepared to send “appropriate armed forces according to the threat,” adding that it was “determined, confident and capable of ensuring national security.”

Pelosi is the highest-ranking American politician to visit Taiwan since then-House Speaker Newt Gingrich did so in 1997. That came shortly after the last major crisis in the Taiwan Strait, when China lobbed missiles into the sea near ports and then-President Bill Clinton sent two aircraft carrier battle groups to the area.

Pelosi will visit Taiwan’s parliament Wednesday morning, have lunch with President Tsai Ing-wen and also meet with democracy activists, according to local media reports. The previously unannounced stop in Taiwan comes after Pelosi led a congressional delegation to Singapore and Malaysia. They will head next to South Korea and Japan—two staunch U.S. allies.

While the White House has sought to dial back rising tensions with China, emphasizing that Congress is an independent branch of government, Beijing has rejected that argument. On Tuesday, Chinese Foreign Ministry spokeswoman Hua Chunying slammed the “provocative” visit and said any countermeasures from Beijing would be “justified.” Still, she left the door open for a possible in-person summit between Biden and Xi later this year.

Taiwan remains the most sensitive issue between the U.S. and China, with the potential to one day spark a military conflict. Biden said in May that Washington would intervene to defend Taiwan in any attack from China, although the White House later clarified he meant the US would provide weapons, in accordance with existing agreements.

Chinese media outlets including the Communist Party’s Global Times have suggested the People’s Liberation Army would respond aggressively to a Pelosi trip.

Under an agreement reached in 1978 to normalize relations between China and the U.S., Washington agreed to recognize only Beijing as the seat of China’s government, while acknowledging—but not endorsing—the Chinese position that there is but one China and Taiwan is part of China.

The U.S. has insisted that any unification between the island and mainland must be peaceful, and supplied Taiwan with advanced weaponry while remaining deliberately ambiguous about whether U.S. forces would help defend against a Chinese attack.

Visits by lower level U.S. lawmakers have also prompted military responses by China. Last November, Chinese warplanes flew around the east side of the island after a visit by a US congressional delegation.