Experts say there’s a problem with President Trump’s plan to build a wall at the Mexican line: Border barriers often don’t work as promised.

That is not deterring the president, who has threatened to shut down the government unless Congress appropriates $5 billion in starter money. And it hasn’t slowed leaders of other nations, either.

In the last 15 years, dozens of big-budget border walls have sprouted up around the world — driven largely by the desire to block the flow of millions of refugees and migrants fleeing war or poverty.

Only about five border walls or fences stood in place at the end of World War II. That number grew slowly, to about 15 at the time of the fall of the Berlin Wall in 1989. Today? About 70, with more planned or under construction.

That's according to research by scholar Elisabeth Vallet, who studies geography, migration and walls at the University of Quebec at Montreal.

“Building walls is a very costly enterprise, and its purpose is mainly electoral and political,” said Vallet, author of Borders, Fences and Walls: State of Insecurity?

Spending those dollars in less-visible programs — such as overseas peacekeeping and climate-change efforts — would foster greater stability in troubled countries, she said, but building a monumental wall “allows the government to show that it is actually doing something.’”

Vallet characterized walls as poor, monolithic solutions to complicated problems, expensive to maintain and ineffective in their purpose: keeping people out. That hasn’t hindered construction in countries from Kenya to Pakistan to Austria. For instance:

  • Hungary, Bulgaria, Slovenia, and Croatia built or expanded border walls and fences during the last four years, seeking to block a million or more war refugees from the Middle East.

  • Israel erected the 436-mile West Bank Wall or “separation fence” in the 2000s as security against terrorism. Palestinians refer to the structure, which stands 25 feet tall in places, as the “Apartheid Wall.”

  • Saudi Arabia began building a 600-mile-long barrier — layers of fences, trenches, watch towers, radar and night-vision cameras — on the Iraqi border in 2015, to forestall attacks by ISIS and other terrorist groups.

  • Norway constructed an 11-foot-high fence at its Russian border in 2016, saying refugees traveling north from Syria were illegally entering the country.

  • In Northern France, a 13-foot-high concrete wall, derisively known as the “Great Wall of Calais,” was built along a highway in 2016 to stop migrants from sneaking onto trucks in efforts to reach Britain.

“This is in direct response to the unprecedented migration of people around the world,” said Ali Noorani, executive director of the National Immigration Forum in Washington. “Countries think they can wall themselves off from the rest of the world, but history has proven otherwise.”

Worldwide, the United Nations estimates, 65.8 million people have been displaced, many by wars in Afghanistan, Iraq, Sudan and Syria. Every day, about 44,000 people flee their homes because of conflict and persecution.

The NIF, which advocates for the value of immigration to the U.S., asserts that even without a wall, the nation’s Southwest border has never been more secure.

The government built nearly 700 miles of physical barriers — with rugged terrain and the Rio Grande acting as natural barricades — since passing border legislation in 2006, the NIF said.

The 16,605 Customs and Border Protection agents stationed in the Southwest in fiscal year 2017 was nearly double the number in 2000. During those same years, Congress nearly quadrupled CBP’s budget, from about $1 billion to $3.8 billion.

What the country needs, NIF says, is an immigration system that promotes security and safety, benefits American workers, and offers legalization to law-abiding undocumented migrants already living in the U.S.

Around the world, walls and barrier fences have met decidedly mixed success.

Hungarian authorities told NBC News that their wall cut off a main migrant route from the Middle East to Western Europe, slashing undocumented-immigration arrests to five in April 2018 compared with 3,528 two years earlier. “I hope it inspires the Americans,” Asotthalom town Mayor Laszlo Toroczkai told the news network.

In Africa, work on a 435-mile Kenya-Somalia border wall was halted in spring because of rising tensions among people living near the line.

The Calais wall has been roundly criticized as useless, easily defeated by migrants who run to the far end of the barrier, loop around, then open security doors to let others cross, according to British media reports.

Trump has made construction of a “big, beautiful wall” a signature element of his hard-line immigration policy, saying it would stop illegal crossings, help keep America safe, and cut crime and drug traffic.

That resonates with the Federation for American Immigration Reform, which seeks much lower levels of immigration. It sees a wall as an integral component of immigration control and national security.

“As we have seen around the world, secure fencing is an effective way to control borders,” president Dan Stein said last year, asserting that the billions spent on a wall would be a fraction of federal, state and local costs incurred due to illegal immigration.

FAIR estimates the cost of undocumented immigrants to American taxpayers at $116 billion a year, though numbers on that issue are highly disputed.

The president won a victory last Monday, when the Supreme Court refused to hear an appeal from conservation groups that challenged his authority to build the wall. The Center for Biological Diversity, the Animal Legal Defense Fund and Defenders of Wildlife contended that construction would harm plants, wildlife habitats, coastal birds and other animal species.

Originally, walls were built as a defense, a means to protect territory.

The Great Wall of China — actually a series of walls, built across hundreds of years and thousands of miles — became obsolete when it was overrun by Mongol raiders. France’s famed Maginot Line, considered an impenetrable barrier of concrete fortifications and obstacles, failed in 1940 when German invaders came up with a novel solution: They went around it.

The cost of Trump’s proposed wall, to line about 1,300 unfenced miles, remains a guessing game. The president put the price at $12 billion. Senate Democrats estimated $70 billion. The Department of Homeland Security says $22 billion.

None of those estimates includes the cost of maintenance. Or a promise of success.

“Walls are not very effective at stopping movement. People can go around, over, under…,” said University of Hawaii geography professor Reece Jones, author of Violent Borders: Refugees and the Right to Move. “The way that walls do work is as a symbol. The material object of the wall stands in for all of the other complex issues about borders, migration, and trade.”

The biggest cost of border walls, Jones said, is the lives of people who die on the move. A tough border defense meant to deter migrants only re-routes them, often to more dangerous and remote passages, Jones said.

In Tucson, Ariz., the hardening of border defenses saw migrant deaths grow from about 18 a year in the 1990s to about 200 a year in the 2000s, he said.

The effectiveness of a wall, he and others said, depends on how well it is guarded. That’s hard to accomplish across hundreds of miles of border, even if a wall looks like a good solution on television.

“A wall is tangible, you can see it, take a picture of it,” Noorani said. “Smart migration policy, smart investment in other countries, that’s difficult. And it takes a long time.”