LONDON — In an apparent effort to bolster his case for a wall along the Mexican border, President Trump pointed on Wednesday to examples of walls going up around the world, especially barriers built in Europe, as proof that they work.Here’s an assessment of those claims.What Was Said “There are now 77 major or significant Walls built around the world, with 45 countries planning or building Walls. Over 800 miles of Walls have been built in Europe since only 2015. They have all been recognized as close to 100% successful.” This is misleading.While governments around the world have built or planned to build 77 walls, about seven such barriers are not expected to materialize anytime soon, said Elisabeth Vallet, a researcher at the University of Quebec in Montreal who keeps a tally of such construction. Seventy walls is closer to the truth, she said. Europe has built about 1,000 kilometers, or just over 620 miles, of walls since 2015, Ms. Vallet said. The White House did not respond to questions about the source of the numbers the president cited in his tweet. Some observers on social media speculated that he had been watching a report on Fox Business that examined the issue the night before.Defining ‘wall’What Mr. Trump may think of as a “big, beautiful wall,” whether made of concrete or steel slats, would not fit the definition in Europe. Most of the walls he referred to on the Continent are, in fact, different styles of fences with masonry foundations. And not all of them were designed with migration in mind. Countries like Austria, Bulgaria, Greece, Hungary and Slovenia have built walls since 2015 to quell the flow of migrants arriving mainly from the Middle East via the Balkan Route. But Ukraine and the Baltic States that share borders with Russia have started raising barriers to mark their territories in response to perceived threats of invasion from their larger neighbor. “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. Walls have worked for politicians who used them as a means to mobilize supporters, but not to deal with the problem of migration, Nick Buxton, a researcher with the institute, said by phone. “You can build more and more walls until you’re a completely walled society, but all you’re doing is making it more and more dangerous,” he said. “People will find ways to cross walls.” The fence around the entrance of the tunnel providing the only land access to Britain from Continental Europe has failed to deter illegal crossings. Last year, about 500 migrants, 10 times as many as in 2017, attempted to cross the perilous waters of the English Channel from France. More than half completed the journey.Reports of smugglers operating around most of these points of access suggest to researchers that walls alone cannot solve the problem. “Every time you have a border fence and no public policy that is dealing with the root problem, it is as if you were inviting organized crime to the table,” Ms. Vallet said. Ms. Vallet said she could not think of historical examples of walls that work. She noted that both the Great Wall of China and the Maginot Line in France — each built at great expense — ultimately failed to stop invaders.