Clinton is suggesting that walls are useless against today’s borderless threats. Obama is suggesting that the world is marching toward ever-more interconnectedness, trampling the walls in its way. Both seem to present walls as a thing of the past. In fact, though, border walls and fences are currently going up around the world at the fastest rate since the Cold War.

This is one piece of evidence cited in a new book on the consequences of connectedness. In The Seventh Sense, Joshua Cooper Ramo argues that in the modern era, the world is in many ways organized into networks that “emerge when nodes—which can be composed of people, financial markets, computers, mobile devices, drones, or any lively and connectable object—link to other nodes.” And the central debate of the networked age—a debate that’s far from settled, despite the confident declarations of Clinton and Obama—is the degree to which these connected systems should be open or closed.

“There are certain reasons networks want to be closed, which have to do with their efficiency,” Ramo told me. “If they’re totally open they become so inefficient that they stop growing. … The essential locus of control in any network system is the role of the gatekeeper.”

Ramo, a former journalist and the co-CEO and vice chairman of the consulting firm Kissinger Associates, applies network theory to international affairs. The fall of the Berlin Wall and the end of the Cold War helped usher in unfettered globalization, he argues, but now a backlash is underway. Globalization has gradually produced a desire in certain parts of the world for separation—particularly after a series of traumas, including the 9/11 attacks and the global financial crisis, exposed the hazards of freewheeling integration. And separation is increasingly being achieved through physical barriers.

The statistic Ramo cites about the spread of walls comes from a study by the political scientists Ron Hassner and Jason Wittenberg: Of the 51 fortified boundaries built between countries since the end of World War II, around half were constructed between 2000 and 2014. Hassner and Wittenberg found that such boundaries—structures like the existing U.S.-Mexico border fence, the Israel-West Bank barrier, and the Saudi Arabia-Yemen border fence—tend to be constructed by wealthy countries seeking to keep out the citizens of poorer countries, and that many of these fortifications have been built between states in the Muslim world.

Number of Separation Barriers Initiated Around the World, 1945 – 2014

Ron Hassner and Jason Wittenberg / The Atlantic

“The walls, fences, and trenches of the modern world seem to be getting longer, more ambitious, and better defended with each passing year,” Ramo writes. “The creation of gates is … the corollary of connection.”

Recently, many of those fences have been appearing in Europe, as countries there struggle to process an influx of migrants and refugees. (The chart above doesn’t account for all of these new barriers, a number of which have been constructed since 2014.) The Economist observed in January that, as a result of the refugee crisis and the conflict in Ukraine, “Europe will soon have more physical barriers on its national borders than it did during the Cold War.” New border controls and barriers, including Austria’s proposed fence along the border with Italy, are threatening the viability of the European Union’s passport-free Schengen zone. Here’s what the situation looked like as of early March: