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For weeks, the pressure has been building. On Tuesday, it appeared to explode. Hong Kong protesters and police clashed in unprecedented scenes of violence in the city’s international airport. Riot police armed with batons and tear gas waded into crowds of protesters massed inside the usually busy international hub. In two separate instances in the airport, groups of protesters had seized men they believed were undercover security agents; one of the men turned out to be a reporter for a Chinese state media outlet.

“The incidents marked a brazen escalation by protesters in confronting those they perceive as infiltrators from mainland China and capped a second consecutive day of chaos at Hong Kong’s airport, among the busiest in the world,” my colleagues reported.

The situation in Hong Kong is nearing a precipice. Neither the protesters nor the Hong Kong government, which is largely controlled by a domineering Beijing, seem willing to cede any ground. On Monday, a senior Chinese official branded the actions of Hong Kong’s dissidents as “signs of terrorism,” while Chinese state media sought to whip up nationalist sentiment on the mainland, casting the protests as the product of foreign or U.S.-backed plots. The actions of the protesters on Tuesday in the airport — they impeded travelers with makeshift barricades of luggage carts, kept a semiconscious plainclothes officer hostage, and brawled with riot police — would probably bolster both the Hong Kong government and Beijing’s insistence that the disturbances are the work of “rioters.”

The specter of a more dangerous clash looms. On Tuesday, President Trump retweeted a video on social media of Chinese troops apparently mobilizing in the megacity of Shenzhen, which is across the border from semiautonomous Hong Kong. In a separate tweet, he limply urged all to “be calm and safe.” Thirty years after the Communist regime snuffed out a student-led democratic uprising in Beijing’s Tiananmen Square, Chinese President Xi Jinping and the rest of the Communist Party leadership must wonder how long they’ll let Hong Kong’s young people continue to stand so defiantly against them.

“We are at a crossroads,” Martin Lee, a venerable Hong Kong advocate for democracy, told the New York Times. “The future of Hong Kong — the future of democracy — depends on what’s going to happen in the next few months.”

Earlier this summer, hundreds of thousands took to the streets to protest an extradition bill under consideration by Hong Kong’s government. Their outcry stalled the legislation but did not secure its full withdrawal. The protests continued and broadened, with a new generation of activists renewing calls for democratic reforms, the defense of the former British colony’s civil liberties and police accountability. Virtually every day, pro-democracy demonstrators have blocked roads, staged sit-ins in public plazas and marched down busy thoroughfares. A general strike last week paralyzed Asia’s financial hub and led to hundreds of flight cancellations.

The protesters have gained energy from the tone-deafness and apparent heavy-handedness of the Hong Kong government’s response to their demands. There’s widespread anger at the brutality displayed by Hong Kong police in their crackdowns on demonstrations; polling shows a majority of Hong Kongers want to see an independent investigation into police conduct. This week, many protesters sported eye patches in solidarity with a protester who lost her eye when shot at close range by what appeared to be a beanbag round fired by the police. The U.N. human rights office on Tuesday said it had “reviewed credible evidence” that Hong Kong law enforcement employed tear gas and other weapons in ways that are “prohibited by international norms and standards.”

In the past, Hong Kong’s authorities have assuaged protest movements with direct political concessions or let them fizzle out on their own. That was the case with 2014′s “Umbrella Revolution,” when the public started to lose sympathy with protesters who had camped for weeks along a major thoroughfare and “occupied” areas close to Hong Kong’s legislature. The protesters, too, grew fatigued and simply lost stamina.

This time, though, there’s little sign of waning momentum. “It could be that the behavior of the police makes people very conscious of what a Hong Kong that is no longer autonomous looks like,” Michael C. Davis, a global fellow at the Woodrow Wilson International Center for Scholars, a D.C. think tank, told Today’s WorldView. He added that he believes “there’s a debate going on within the elites in Beijing, and you could guess that Xi could feel the need to show a hard line on this and take action, but the costs would be high.”

For China’s leadership, the doggedness of Hong Kong’s protesters presents a real challenge. “On the one hand, Xi can’t let this drag on too long — he is pursuing the so-called China Dream to make China great again,” Warren Sun, an expert on Chinese politics at Monash University in Australia, told my colleague Gerry Shih last week. “On the other hand, he needs to be very careful because the international world, including Trump and Taiwan, are watching to see if he mishandles things.”

In an essay this week, Minxin Pei, a professor of government at Claremont McKenna College, spelled out how poorly a military crackdown would go for China’s leadership: “Hong Kong’s residents would almost certainly treat Chinese government forces as invaders, and mount the fiercest possible resistance. The resulting clashes — which would likely produce high numbers of civilian casualties — would mark the official end of the ‘one country, two systems’ arrangement, with China’s government forced to assert direct and full control over Hong Kong’s administration.”

Pei added that “with the Hong Kong government’s legitimacy destroyed, the city would instantly become ungovernable. Civil servants would quit their jobs in droves, and the public would continue to resist.” This, in turn, would have devastating consequences for the city’s economy, and — given its place as a global logistics and financial hub — that of the world, to boot.

So the prospect of a Chinese “invasion” is, for now, still distant. “I believe they have learned the lesson that the price of using the military is very high,” Wu’er Kaixi, one of the leaders of the 1989 Tiananmen protests, told Agence France-Presse over the weekend.

But that doesn’t mean it’s off the table. “I don’t think there are people in the Chinese leadership who believe that Tiananmen was a mistake,” Bonnie Glaser of the Center for Strategic and International Studies told Today’s WorldView. She added that Trump, who has essentially waved away interest in Hong Kong’s unrest, may have signaled “to the Chinese that they can intervene with impunity.”

For now, though, authorities in Beijing and Hong Kong may hope other tactics work. “The Chinese and the government in Hong Kong are trying to come up with creative ways short of use of real force,” Glaser said. They may keep infiltrating the protests with their own personnel, while hoping the deepening radicalism of hardcore activists gradually turns the wider Hong Kong public against them. They are also stepping up a propaganda campaign in a bid to cool enthusiasm for the Hong Kongers’ demands abroad. The protesters, meanwhile, would probably treat the resignation of Carrie Lam, Hong Kong’s leader, as a huge victory, but that doesn’t yet seem to be in the cards.

And so the struggle continues. In Hong Kong’s airport, some caught in the middle look on with sympathy. “I can’t blame anyone,” Krishna Hariharan, a 27-year-old Indian engineer marooned on holiday, told my colleagues. “They are seeking justice, and it just happens that our fates are intertwined like this. If the government comes down hard on them — then what are they governing for?”

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