The new wall takes many forms - virtual and physical, high tech and low tech. This has ranged from tighter visa restrictions and carrier sanctions to expanding border guard forces and physical barriers. In the case of Germany, the new wall has even been propped up by those guarding the old wall: many former East German border guards have been incorporated into a mushrooming federal border force, this time with the mission to keep people from entering rather than leaving. Germany’s border police more than tripled in size in the decade after the Berlin Wall came down. To deter “undesirables,’’ Germany has also dramatically tightened its asylum laws - forcing some would-be asylum seekers to try more clandestine entry methods. Germany has also been the most forceful advocate of “Europeanizing’’ border controls, creating a shared EU space (“Schengenland’’) with a fortified outer perimeter (dubbed “fortress Europe’’).

This ambitious wall-building project has also used neighbors as buffer zones, including the signing of readmission agreements assuring that neighbors take back migrants who use their territory to scale the immigration wall. As part of the price of EU entry, Poland hired thousands of border guards, built more border stations, purchased new equipment, and implemented tough new laws against unauthorized migration. As Maciej Kuczynski, the deputy director of the Polish department of Migration and Refugee Affairs, candidly explained: “If we want to integrate into the European Union, we have to show our goodwill in fighting illegal immigration.’’

Meanwhile, Europe is in a state of demographic denial. Countries such as Germany, Italy, and Spain need an influx of migrants to stem plummeting population declines. UN projections are startling: Italy is expected to shrink by over 16 million people by 2050, and Spain and Germany are expected to shrink by nearly 10 million each. Germany needs 500,000 new migrants per year simply to maintain the size of its current economically active population.

Nevertheless, domestic political pressure has translated into escalation rather than re-evaluation. While the Berlin Wall provoked public outrage, public outrage in Europe today pushes for a higher and wider immigration wall. And with the front door more tightly closed, the only way in is increasingly through the back door - overstaying visas, fraudulent use of documents, and hiring smugglers. Roughly 500,000 unauthorized migrants enter the European Union annually. Thus, the immigration wall is not impenetrable, but pushes migrants into the shadows and criminal underworld.

The United States has similarly embarked on an ambitious wall-building project along its southern border with Mexico, the world’s most dramatic dividing line between rich and poor. Cold War leftovers - 180,000 metal sheets used for landing mats for military operations -provided the initial wall-building material in the early 1990s. Mexicans dubbed it the “new Iron Curtain.’’ What started out as a few miles of metal barriers now extends for hundreds of miles, with portions of it triple-layered. New physical barriers have been matched by an influx of new equipment and personnel. The size of US Border Patrol doubled in the 1990s, and has doubled again this decade. This has predictably created a booming business for professional smugglers, as migrants have become more dependent on smugglers to get around, under, or over the border wall.

The expanding wall along the southern border has so far shifted clandestine entries away from urban areas to more remote and treacherous desert and mountainous terrain. This has also made the immigration wall more lethal than the Berlin Wall, with hundreds of border crossers dying every year.

Despite all the lofty business school rhetoric about globalization creating a “borderless world,’’ border barriers are hardening when it comes to migration. The triumph of the freedom of exit that the fall of the Berlin Wall so potently symbolized has increasingly become meaningless for the vast majority of the world’s inhabitants who lack freedom of entry.

**Peter Andreas, author of “Border Games: Policing the US-Mexico Divide,’’ is associate professor of political science and international studies at Brown University, where he also directs the international relations program.