House Speaker Nancy Pelosi left Taiwan Wednesday after a visit that inflamed US tensions with China — which is responding with its largest military drills in more than 25 years in what experts warned could be “seen as an act of war.”

The defiant 82-year-old California Democrat and her delegation flew out of Taipei after a roughly 19-hour visit that was the first by a House speaker in 25 years.

Pelosi’s trip had so enraged mainland China that the country’s most popular social media platform, Weibo, crashed for about 30 minutes, confirming it was overstretched as several hashtags racked up several billion views.

“This old she-devil, she actually dares to come!” popular blogger Xiaoyuantoutiao wrote, adding that they had gone to bed “so angry I could not sleep.”

But the online anger was nothing compared to that of the Beijing government, which views Taiwan as part of its sovereign territory and had made a series of ominous threats while making clear that the speaker’s visit would be considered a major provocation.

Even before Pelosi’s arrival, Chinese warplanes buzzed the imaginary line dividing the Taiwan Strait, with the People’s Liberation Army saying it was on high alert and would launch “targeted military operations.”

After Pelosi landed, China announced that it would hold four days of “necessary and just” joint air and sea drills beginning Thursday, the largest aimed at Taiwan since 1995.

The drills would include live fire and test launches of conventional missiles, according to the Chinese state news agency Xinhua.

At least half of the six areas where the drills are planned to take place appear to infringe on Taiwanese waters, according to Arthur Zhin-Sheng Wang, a defense studies expert at Taiwan’s Central Police University.

Using live fire in a country’s territorial airspace or waters “can possibly be seen as an act of war,” Wang warned.

“Such an act equals to sealing off Taiwan by air and sea … and severely violates our country’s territorial sovereignty,” Taiwanese Capt. Jian-chang Yu said at a briefing by the National Defense Ministry.

Taiwanese President Tsai Ing-wen insisted Wednesday that the island of 23 million would not be cowed.

“Facing deliberately heightened military threats, Taiwan will not back down,” Tsai during her meeting with Pelosi.

“We will firmly uphold our nation’s sovereignty and continue to hold the line of defense for democracy.”

However, Beijing’s Foreign Ministry insisted: “In the current struggle surrounding Pelosi’s Taiwan visit, the United States are the provocateurs, China is the victim.”

Chinese Vice Foreign Minister Xie Feng also summoned the US ambassador in Beijing, Nicholas Burns, to lodge formal protests of Pelosi’s visit, while China banned some imports from Taiwan, including citrus fruit and fish.

Through it all, Pelosi remained defiant, saying she was there to send the “unequivocal message: America stands with Taiwan.”

“Today the world faces a choice between democracy and autocracy,” she said in a short speech during the meeting with Tsai.

“America’s determination to preserve democracy, here in Taiwan and around the world, remains ironclad.”

Pelosi’s delegation included Reps. Gregory Meeks (D-NY), chair of the House Foreign Affairs Committee, and Raja Krishnamoorthi (D-Ill.) of the House Intelligence Committee.

The delegation’s next stop will be South Korea before wrapping up its Asian tour in Japan. The lawmakers previously visited Singapore and Malaysia before the unannounced stop in Taiwan.

With Post wires