China’s president, Xi Jinping, is confronted here by a challenge that he can’t easily steamroll, and there’s no obvious exit ramp for either side. And if Xi can’t manage to keep Hong Kong content, how can he dream of unifying with Taiwan?

So look out. Xi is intolerant and overconfident, thus one risk is that he will ultimately deploy the People’s Armed Police, a paramilitary force, to crush the protests. Alternatively, he can use Hong Kong’s triad gangsters — who have a long history of working for Beijing — to use cleavers and baseball bats to terrorize or eliminate democracy activists, even as the Hong Kong police step up their own arrests. We should be alert not only for troops pouring across the border, but also for thugs with cleavers.

One concern is that Xi may make decisions based on bad information relayed by sycophants. In my conversations here, I sense that pro-China officials actually believe that the protests were orchestrated by American and Taiwan officials, and it’s always dangerous when dictators become persuaded by their own propaganda.

In addition, Chinese news coverage of the protests has resulted in a wave of anti-Hong Kong nationalism on the mainland that may add to the pressure to suppress protesters. In fairness, mainlanders do make one valid point: If protesters in America were throwing Molotov cocktails at police officers, the United States might well have seen more than the single warning shot fired by the police so far in Hong Kong.

This great city may be at a turning point, and Trump and other world leaders should make clearer that Xi will pay a severe price if he uses force — whether by troops or by triads — to try to crush Hong Kong. Granted this must be done delicately, because publicly siding with protesters risks confirming Xi’s narrative that America is secretly steering the movement.

One helpful step would be for Congress to pass the bipartisan Hong Kong Human Rights and Democracy Act, which would sanction officials who suppress freedoms in Hong Kong.

Hong Kong is one of the great cities of the world, and I have been in love with it ever since I lived here in the mid-1980s as a foreign correspondent for The Times. This city is resilient, and no one has ever made money for long betting against Hong Kong. But today it is at risk.