Here is a scenario I find all too plausible: President Trump drops his surprisingly laissez-faire approach to coronavirus and shifts authoritarian. He uses the threat of a pandemic to severely restrict international travel to the United States and institutes stringent new limits on U.S. citizens' ability to travel abroad — perhaps mandatory medical testing, biometric data collection, or vaccination before you leave; extended interviews with Customs agents to vet the reasons for your trip; or even a near-complete embargo on visits to China and other countries where coronavirus outbreak coincides, in Trump's mind, with an economic threat to America. ("You never want a serious crisis to go to waste," after all, because "it's an opportunity to do things that you think you could not before.")

And then, a few years down the line, after the worst of coronavirus has passed, the new rules simply never go away.

Civil libertarians like me would raise a fuss, certainly, but we raise lots of fusses about lots of things (TSA, NSA, etc.) to no real end, because once the state claims new power over our lives, it is rarely relinquished. Prevention is always more feasible than reform — which is why, as coronavirus spreads, we should have an eye to stopping our freedom of movement from being permanently curtailed.

This would not be the first time a crisis has been used to fundamentally change global mobility: Just over a century ago, the passport system we now take for granted did not yet exist. In the decades before the first World War, international travel was generally unhindered. Travel papers had been required in some places in the past, but the Industrial Revolution led to a trend of travel liberalization in the 19th century. A train might cross multiple borders in Europe within a single day, and the logistics of controlling border crossings became overly complicated. Consequently, travelers could typically travel whenever, wherever, and for whatever reason they wanted.

During World War I, the Triple Alliance nations (Austria-Hungary, Germany, and Italy) began requiring passport use, and both Triple Entente states and neutral countries soon followed suit. After the war, the United Nations' precursor, the League of Nations, was tasked with making "provision to secure and maintain freedom of communications and of transit" for all member states. Thus, in October 1920, the League held the first passport conference in Paris. Its aim, as Speranta Dumitru, a professor of political science at the University of Paris Descartes, explained in a 2016 history of passports, "was to restore the pre-war regime of freedom of movement." That aim was not achieved.

Participants agreed transit was "hindered by passport and Customs formalities," as it said in the resolution adopted, and these "difficulties affecting the personal relations between peoples of various countries constitute a serious obstacle to the resumption of normal intercourse and to the economic recovery of the world." But while the resolution made some reforms, Dumitru writes, rather than abolishing passports, it established "a uniform, international passport, issued for a single journey or for a period of two years."

The 1920 resolution also expressed hope for further deregulation in the future, but that never happened. Instead, the uniform passport became the basis for the system we've had ever since. Further attempts to return to pre-war travel freedoms also failed, Dumitru documents, in 1924, 1926, 1947, and 1963. Today, the cause of passport abolition has fallen right out of the Overton Window. We've become so accustomed to carrying papers when we travel that the debate has moved on to whether we must carry them all the time.

Permanent, serious strictures on our freedom to travel may seem unlikely at the moment. This is the age of tourism, with international arrivals nearly tripling between 1995 and 2018 alone. Air travel demand has been booming. Even with the added bureaucratic limits of passports and visas, modern means of transport allow the average person to travel more often and easily than royalty in eras past. And wouldn't such repressive measures, particularly those affecting U.S. citizens, be widely unpopular?

Yes, but what would people do about it? March in the streets? That doesn't have a great track record with this administration. Did the Women's March, the Science March, or any of the other big anti-Trump gatherings actually change any policy? Do we have any reason to think a post-coronavirus March for Tourism would do better?

And anyway, passports were once widely unpopular, too. The 1926 passport conference observed that the public desired "a resolution contemplating the abolition of passports at the earliest possible date." It was a desire unmet.

Public health experts say travel restrictions and large-scale quarantines are typically ineffective and come with serious drawbacks, though sometimes, depending on the type of disease, targeted quarantines can help contain the spread of infection. Yet as the coronavirus crisis is used to curtail international travel — there's already a ban on U.S. entry by foreign nationals who have been in China in the previous two weeks and a mandatory quarantine for U.S. citizens returning from Hubei province — we would do well to take care from the outset that measures are as minimal as possible and firmly bounded by expiration dates.

Draconian travel restrictions are not an inconceivable outcome here, and, if history is any guide, once done, they won't be easy to undo.