What a border means depends on who you are. The reopening of international borders after Covid lockdowns was hailed as a return to normality, at least for wealthy global travellers. At the same time British politicians, crowing about having “taken back control of our borders” after leaving the EU, set about the surreally punitive wheeze of outsourcing asylum to Rwanda. For the fortunate, a border might be merely a queue at an airport; for those less so, a literal wall between their home and workplace.

Borders have often been figured as the skin of the body politic: a defence against external infection. So 19th-century theorists such as Friedrich Ratzel argued that, just as an organism’s skin can stretch to accommodate its growth, so can geographical borders, shifting outward to give the nation adequate room to live, or “Lebensraum”. We know where that led.

Some Russia-watchers, meanwhile, suspect that Vladimir Putin, currently attempting to redraw borders by violence in Ukraine, spent lockdown poring over maps of the old Russian empire. Russia’s equivalent of Google Maps, Yandex, announced in June that it was removing all borders from its app, supposedly to switch to an emphasis on “natural objects”. “Imagine there’s no countries,” as the song goes. “It isn’t hard to do.”

Many national borders, of course, follow the course of “natural objects” such as rivers or mountain ranges. But these are not as reliable as they once seemed, as one of the chapters in James Crawford’s richly essayistic travelogue explains. The border between Italy and Austria is moving every year because the glacier that defines it is melting. One interviewee relates how he penetrated an inner bureaucratic sanctum to discover that Italy’s borders with Slovenia, Austria, Switzerland and France are all defined on official documents held in three sturdy filing cabinets, slowly going out of date as the climate shifts.

In George RR Martin’s Game of Thrones, members of the Night’s Watch guard a vast wall separating the kingdom from the snowy wilderness to the north, and its dangerous inhabitants. So too did Roman soldiers once man Hadrian’s Wall, and later the Antonine Wall. Our author visits the latter and remarks: “This was bordering as a grand performance […] a 37-mile-long symbol of the Roman ability to tame wilderness and nature.”

But why build a wall at all? In Crawford’s account, the Romans’ desire to define a physical limit to what once had been thought of as a limitless empire arose from a crisis of insecurity. The same could be suggested of the electronic walls that fence in citizens of authoritarian countries in an attempt to block access to the wider internet. Keeping the barbarians out is one function of borders; another is keeping the hostage-citizenry in. The Great Firewall of China, and Russia’s equivalent, implement a kind of epistemological quarantine.

The Alien films starring Sigourney Weaver are all about quarantine, and the consequences of disrespecting the border between ship and the external unknown. (If everyone had listened to Ripley there would never have been any trouble with space monsters.) And so the ultimate border is that between the organism and its environment, as Crawford discusses in an excellent chapter on the Covid pandemic. Your body, he reminds us, “is a landscape that is under constant attack and always has been. A landscape that never stops watching its perimeter for incursions or unauthorised entry.” Scientists studying pathogens must work in “biosafety” certified labs with impermeable borders to the outside world. Covid-19 in particular causes such havoc because it triggers an overreaction of the immune system. As Crawford comments darkly: “The system becomes fixated on the entry of foreign bodies, and it tears itself apart trying to keep them out. It is tempting to suggest that there is no virus more appropriate for our current age.”

Not unrelated in its effects is the so-called separation wall built by Israel, which initially tried to get everyone to call it a “fence”. A glimmer of light relief is provided by Crawford’s observation of a graffito on the barrier: “Make hummus not walls.” He buys a can of spray paint and contributes his own graffiti: a stencilled recreation of the Sumerian phrase for “no man’s land”. The sociologist Baha Hilo explains to the author the wall’s effect. “This wall doesn’t really separate Palestinians from Israelis, you know,” he says. “Because there are Palestinians and Israelis on one side and Palestinians and Israelis on the other side. But the wall is an obstacle. Is it an obstacle for a Jewish Israeli person? No. A Jewish Israeli doesn’t experience a checkpoint. They greet you, give you a nice wave. That is a checkpoint for them. As a Palestinian it is something else. The wall is an obstacle between Palestinians and Palestinians.” The great Israeli novelist Amos Oz, for his part, has written that it is time for his country to “finally awaken from the hypnosis of the map”.

Without any borders at all, mind you, there would be no way of distinguishing “here” from “over there”, and political theorists tend to agree that national borders are essential for welfare states to work. Even traditionally stateless peoples, such as the northern Scandinavian Sami interviewed by Crawford, can welcome borders insofar as they bring legal protections and fishing rights.

Might we be nonetheless able to reimagine borders as non-violent, even pleasant things, like the borders of gardens? There is the beginning of a “Great Green Wall” of trees across the continent of Africa, designed to combat creeping desertification. Crawford’s star witness here is a Cameroonian campaigner for agroforestry called Tabi Joda, a scholarly ecowarrior who is sceptical of planning from afar in airy UN offices but nonetheless insists on the possibility of a better future. “As Africans,” he says, “we are living within lots of artificial borders, imposed borders. And I feel really not just Africa, but I think the entire world needs to redefine what we call borders. The whole world should see itself as an entire ecosystem.” As Lennon sang, it’s easy if you try.

• This article was amended on 23 August 2022. The guards of the wall in Game of Thrones are called the Night’s Watch, not the Black Watch.

• The Edge of the Plain: How Borders Make and Break Our World is published by Canongate (£20). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply.