The Trump administration is denying passports to U.S. citizens who live in Texas near the U.S.-Mexico border, according to news reports.

The administration is accusing applicants of having inadequate documentation of their birth on U.S. soil, and refusing to issue them passports on that basis.

Critics argue this is part of a tide of anti-immigrant measures that includes other Trump administration efforts to restrict entry to the U.S. Those measures range from the travel ban on Muslims from certain countries entering the U.S. to White House proposals to develop a merit-based immigration system.

Meanwhile, the entry of thousands of immigrants and refugees into Europe in recent years has generated a populist backlash against outsiders.

These developments raise fundamental questions about migration from country to country: When and how did governments get the power to limit people’s movements? And how did passports come to play such a crucial role?

I explored these questions in the research I did for my book, “The Invention of the Passport.” I believe this history can help us understand how governments have assumed so much control over where people can go.

Moving around

Throughout much of European and American history, labor was forced. Both landowners and states sought to restrict the movement of slaves and serfs in order to prevent the loss of their labor forces. Before the 19th century, however, their ability to keep people from leaving was tenuous and a major source of concern for their owners. In the United States, patrols helped enforce fugitive slave laws, but their reach was limited.

Nobles, merchants and free peasants may have moved about freely, but could be shut in or out of a city in an emergency if the gates were closed.

Until fairly recently, stopping people from leaving a plantation or farm was more important to governments than keeping people from coming in, at least during peaceful times.

That changed following the French Revolution, which began in 1789. Nationalism – the idea that particular “peoples” or “nations” should govern themselves – became a powerful force in Europe and, gradually, around the world. By the middle of the 19th century, both U.S. slavery and European serfdom declined as a result of rising notions of “free labor” and the desire to make populations feel a sense of belonging to the country. The shift toward free, mobile labor meant people had more opportunity than ever to move around.

There were major exceptions: By the early 20th century, the overwhelming majority of states in the world were still authoritarian or colonial. People who lived there could not freely move about.

However, after World War II and the gradual breakup of colonial empires, moving within countries came to be widely understood as a matter of individual freedom. Such movement facilitated the ability of laborers to go where they were needed, and thus tended to be supported by governments.

People leaving a country might still have been regulated by their government in the post-war era. But this became less of a concern because democracy spread. More democratic countries were less worried about people leaving than were those that forced their populations to stay and work, such as those “behind the Iron Curtain.”

It was control over the entry of outsiders that became paramount with the mid-20th-century triumph of nation-states. Foreigners, the thinking goes, might not have the interests of “the people” at heart. A kind of permanent suspicion took hold in which foreigners were deemed ineligible for entry without evidence that they would not become troublesome. Possession of a passport helped promote that by showing who a person was and where they could be sent if they proved undesirable.

As I argue in my book, this transformation in regulating movement created a new world that would be largely unrecognizable to those who lived before World War I. Governments everywhere now restrict during peacetime the entry of people they deem “undesirable” on criminal, ethnic, economic, medical and demographic grounds.

Meanwhile, movement within countries loosened up, although particular spaces – such as military bases, prisons and areas containing valued resources – often remain off limits to many.

Since then, crossing international borders has become the big challenge for people wishing to move. Passports became key to regulating this process.

Papers, please

Passports, seemingly modest documents, were introduced gradually in many places in the modern world. In the United States, the federal government in 1856 asserted the exclusive right to issue passports and mandated that they be issued only to U.S. citizens.

Once simple pieces of paper, passports have evolved into standardized booklets that identify persons and tell governments where they should be sent if they are deemed inadmissible – their fundamental purpose in international law.

Today, passports are perceived mainly as documents that are used to constrain entry into a country, weeding out the relatively rare individual who might be a criminal, a terrorist or someone otherwise at odds with the receiving government’s preferences.

Since the 9/11 terrorist attacks, governments have developed a greater interest in technological means of identifying border-crossers. For example, governments that belong to the standard-setting International Civil Aviation Organization have developed machine-readable passports with encrypted identification information, making them harder for anyone to use other than the actual bearer.

Those whose movements are being scrutinized so intently today in North America and Europe are from countries whose citizens are often regarded as undesirable due to poverty, culture, religion or other attributes. Entry of these outsiders has generated a wave of support for nationalist, populist parties that are upending the traditional openness to foreigners in the United States and fuelling xenophobia in Europe.

By challenging the passport applications of people born near the Mexican border, the Trump administration is also reminding us that passports are a reflection of one’s citizenship. Without one, you can’t leave the country and count on being able to return. Their freedom to remain in the U.S. is at risk.

We live in a world in which the entry of those who are deemed “desireable” is greatly facilitated, while that of those deemed “undesirable” is greatly constrained. Freedom of movement into other countries is a reliable expectation only for those from the rich world with no blemishes on their records; for the rest, crossing borders can be very difficult, indeed.

John Torpey receives funding from the German Marshall Fund.