“Almost a third of the walls in the world are designed to keep the neighbor out,” Ms. Vallet said.

‘Fortress Europe’

Much like the desert terrain along the United States border with Mexico, the perilous seas, not walls, have been the main obstacle for people trying to get to Europe.

The Continent has gone beyond building walls to impede the arrival of undocumented migrants. Frontex, the European Union’s border agency, and national governments raised a security apparatus that is often referred to as Fortress Europe.

It includes naval patrols and surveillance on the Mediterranean, where most migrants try to cross into Europe. European governments work with countries like Libya, Morocco and Turkey to try to deter migrants from attempting deadly sea crossings.

Last year, an estimated 150,000 illegal crossings were detected on the external borders of the European Union, according to Frontex. (The same person may attempt the crossing and be counted in different locations.) This was 25 percent less than in 2017, and the lowest level in five years.

Still, far-right politicians like Prime Minister Viktor Orban of Hungary have promoted the specter of an impending migration crisis, accompanied by xenophobic rhetoric, to justify funds for border protection.

[Read more about whether Europe is still facing a migration crisis.]

Success is relative

“The boosting and militarization of border security has led to a higher death toll for forcibly displaced persons,” according to a report released in May by the Transnational Institute, a research and advocacy institute.

In another report on border walls, researchers at the institute looked at maritime barriers in the Mediterranean and considered them as significant or more significant than land barriers on the European continent.