"We must save Hong Kong, the present Hong Kong and the future Hong Kong," declared the city's chief executive, Carrie Lam, in a Friday announcement of a ban on wearing masks in public. Enacted under a colonial-era emergency powers ordinance, the mask ban takes effect Saturday and is ostensibly a "deterrent to radical behavior," a way to tamp down the rising violence from a minority segment of the anti-government protesters who have flooded the city's streets for weeks.

Yet, as demonstrators have been quick to note, punishing the use of face coverings with jail time will do far more than make the violent few easier to apprehend. It will also expose thousands to tear gas employed by increasingly brutal police, and it bares protesters' identities to China's panopticon surveillance state. Hong Kong is right to bristle at this ban — and we in the United States would do well to take it as a warning. The civil liberties violations now underway in China can happen here, and they almost certainly will if we do not proactively reject them.

In some arenas, the march of technological development does not substantively change the questions policymakers must address. Civil liberties are different. A century ago — even a few decades ago — the surveillance capabilities now available to the government were the stuff of science fiction. Imagine telling someone in 1919 or 1979 that in some cities the state can tap into thousands or even millions of cameras to spy on citizens.

Chinese cities top the global list of camera concentration, with as many as 168 cameras for every 1,000 people, per a recent study. And this is not the grainy, black-and-white footage of a gas station security cam. Chinese researchers have built a camera capable of taking images five times more detailed than those perceived by the unaided human eye. Paired with facial recognition technology, it will be able to identify individuals in crowds of thousands. China's facial recognition tech will soon gatekeep much of the country's internet usage, too, as a facial screening will be required to obtain new cell phone service beginning in December.

This degree of spying is difficult to fathom, and it is not hyperbolic to say it is a legitimately new development in state power. Think of our stories of heroic outlawry, historic and fictional alike: Robin Hood, the Scarlet Pimpernel, the Underground Railroad, resistance to the Nazis in occupied Belgium and France. Would any of that be possible now? Can Robin give the sheriff the slip when his escape is caught on camera? How do you run the Underground Railroad under the watchful eye of CCTV? How do you resist a tyranny that can listen to your every word and watch your every move?

I'm not sure we've grasped the extent to which this technology makes preemptive limits of state authority vital. It is inherently and perhaps uniquely self-preservative. The omniscience of which modern surveillance tech is capable gives it a built-in omnipotence. By its nature, it makes resistance extremely difficult. Other forms of state oppression may be more obviously threatening, but pervasive surveillance has a special quality of self-enforcement.

That quality is what makes the warning from China so urgent. The technology available to Beijing is available to Washington, and the lust for power which encourages its employ knows no national bounds. What is happening there absolutely can happen here. Chinese cities lead the list of surveillance camera concentration worldwide, yes, but Atlanta and Chicago make the top 20, and Washington, D.C., San Francisco, San Diego, and Boston are in the top 50. Facial recognition use is growing among American law enforcement, too, including at the federal level.

Our situation and that of the demonstrators in Hong Kong (and Chinese citizens more broadly) are not as far apart as we might like to think. All that really stands between the two is law.

That is why the civil liberties crackdown in Hong Kong, of which this mask ban is but a portion, should prompt us here in the States to get our legal house in order. We must strengthen privacy protections before we have finished assembling a panopticon of our own. The surveillance state suppresses the very dissent it occasions. Once the cameras go up, they are exceedingly unlikely to come down.

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