Big promises are to be expected from presidential candidates, but reality often intrudes. The elder George Bush broke the “no new taxes” pledge that helped lead to his election. And Barack Obama’s administration has yet to live up to his prediction that his nomination would go down in history as the moment “when the rise of the oceans began to slow and our planet began to heal.”

Donald Trump’s vow to restore what he says is America’s lost luster, while perhaps not as flowery, comes with campaign promises that are equally grandiose. But Trump, the presumptive Republican nominee, has typically provided scant details on how he might make good on his promises — and ambitious ideas, even the concrete kind, do not always add up.

Central to Trump’s campaign, and to his national security strategy, is his intent to clamp down on illegal immigration, using a vast deportation “force” to relocate people to the other side of a wall, funded by Mexico, that would stretch nearly the length of the southern border.

Trump has suggested he will flesh out his ideas in a forthcoming speech. But experts across many fields who have analyzed his plans so far warn that they would come at astronomical costs — whoever paid — and would in many ways defy the logic of science, engineering and law.

Mass deportations

Trump has a simple plan to reduce the population of 11 million immigrants living illegally in the United States: Deport them.

How? He says he would follow the example of the military-style roundups authorized by President Dwight D. Eisenhower in 1954. The initiative, known as Operation Wetback, expelled hundreds of thousands of Mexicans.

Trump contends that the start of deportations would show immigrants he meant business and prompt many to leave on their own, and that it would take about two years to finish the job. There, the specifics end.

Former senior immigration and border officials are skeptical, to put it mildly. Deportations have peaked recently at about 400,000 a year, so the increase in scale to reach Trump’s goal would be exponential. And many legal procedures and constitutional constraints on the police did not exist in the Eisenhower era.

“I can’t even begin to picture how we would deport 11 million people in a few years where we don’t have a police state, where the police can’t break down your door at will and take you away without a warrant,” said Michael Chertoff, who led a significant increase in immigration enforcement as the secretary of Homeland Security under President George W. Bush.

Finding those immigrants would be difficult, experts said. Police officers across the country would need to ask people for proof of residency or citizenship during traffic stops and street encounters. The Border Patrol would need highway checkpoints across the Southwest and near the Canadian border. To avoid racial profiling, any American could expect to be stopped and asked for papers.

To achieve millions of deportations, the Obama administration’s focus on deporting serious criminals would have to be scrapped, said Julie Myers Wood, a director of Immigration and Customs Enforcement, also known as ICE, under Bush. “You would not care if the person had a criminal record,” she said.

Large-scale raids, rare under Obama, would resume at farms, factories, restaurants and construction sites, with agents arresting hundreds of workers and poring over company records. And prosecutors would bring criminal charges against employers hiring unauthorized immigrants.

Trump has said he would triple ICE’s deportation officers, to 15,000 from about 5,000. But even if that could be accomplished quickly — difficult given the vetting and training required — it would still be insufficient, experts said. The FBI and other agencies would have to set aside some of their missions to help.

John Sandweg, who led ICE for seven months under Obama, said wholesale deportations could make it easier for immigrant gang members and drug traffickers to escape detection. “If the agents are looking for volume, they won’t spend the time to do the detective work tracking down the high-value bad guy who has fake documents, the hardened criminals in the shadows,” he said.

To prevent flight after arrest, the authorities would have to detain most immigrants awaiting deportation. Existing facilities, with about 34,000 beds, would have to be expanded to hold at least 300,000, Sandweg estimated, perhaps with tens of thousands of people in detention camps, similar to the internment of Japanese-Americans during World War II.

Most deportations must be approved by judges. But backlogs in the 57 immigration courts are already severe, with waits as long as two years for a first hearing. The federal government would have to open dozens of emergency courts and hire hundreds of judges, shortcutting the painstaking selection process.

The millions of immigrants from Central American countries, China, the Philippines, India and other noncontiguous nations would have to be flown home at the federal government’s expense. Arranging flights would in itself be a huge and very costly task.

At the border, where illegal crossings have fallen to the lowest levels since the 1970s, the Border Patrol would face a rush of newly deported immigrants trying to return, experts said.

“Many of these people have been here for decades,” said David V. Aguilar, who was the chief of the Border Patrol and then the commissioner of Customs and Border Protection. “We should expect an immediate effort to reunify with the family left behind.”

Aguilar said a surge of returning deportees could divert the Border Patrol from the fine-tuned hunt for terrorists trying to infiltrate the country, now its top priority.

Wood said mass deportations would add chaos to a dysfunctional immigration system.

By any tally, the costs would be enormous. The American Action Forum, a conservative-leaning research group, calculated the federal outlay to be at least $400 billion, and then only if the deportations were stretched over 20 years.

But the proposals’ main flaw, former officials said, is that they are unrealistic. “Unless you suspend the Constitution and instruct the police to behave as if we live in North Korea,” Chertoff said, “it ain’t happening.”

The wall

Trump has promised that the wall will be big, beautiful, tall and strong. Spanning 1,000 miles along the southern border, it will stem the flow of immigrants bringing drugs and crime. And, yes, Mexico will pay for the Great Wall of Trump, as he has called it.

But the wall — symbolic of an iron-fisted immigration policy and providing a rallying cry for his supporters — has proved to be as divisive in theory as it would be in practice. And experts in domestic security, immigration policy and civil engineering say that building it would be a daunting task and cause more problems than it would solve.

Trump has shared few details. He has said that the wall would be built from precast concrete and steel and that it could be 50 feet tall, if not higher. After calling for it to extend across the entire 2,000-mile southern border, he more recently said half that length could be sufficient because of natural barriers. He has pegged the cost at $4 billion to $12 billion, most recently settling on around $10 billion.

Some see that as low. “There’s a lot of logistics involved in this, and I don’t know how thoroughly they’ve thought it out,” said Todd Sternfeld, chief executive of Superior Concrete, a Texas-based builder of walls. “The resources alone would be astronomical.”

Sternfeld, who has led major wall projects across the country and approached the Trump family last summer, suggested that Trump was overly optimistic about the cost and was underestimating the complexity of the undertaking.

Running the numbers, Sternfeld said a 40-foot-tall concrete wall using a “post and panel” system that went 10 feet below the ground — to minimize tunneling — would cost at least $26 billion. The logistics would be nightmarish, including multiple concrete casting sites and temporary housing for a crew of 1,000 workers if the job were to be completed within Trump’s first four-year term.

Maintenance would be an additional recurring expense, said Walter W. Boles, an engineering professor at Middle Tennessee State University who specializes in concrete construction. Deep trench work would also be necessary for keeping a wall of that height from toppling, he said, and seismic sensors to detect digging would be wise for preserving its integrity from below.

“That’s one heck of a construction project,” said Boles, who assessed Patrick J. Buchanan’s 1996 proposal for a border barrier. “It’s certainly a lot more ambitious than I was imagining.”

The most common benchmark used for assessing Trump’s wall is the fencing that already exists at the border. In 2006, the Bush administration signed the Secure Fence Act, and $2.4 billion was spent to construct 670 miles of fencing over three years, according to a 2009 Government Accountability Office report. Many Republicans, including Trump, have argued that the fence turned out to be too porous, with much of it designed to keep out only vehicles, not pedestrians.

Thad Bingel, the chief of staff at Customs and Border Protection during the Bush administration, said the fence offered a cautionary tale. It became mired in lawsuits and environmental controversies that sowed deep anger along the border, and required the extensive use of eminent domain, a practice that Trump has been criticized for using to seize private properties for his big developments.

A wall would be even more complicated, requiring redirection of water so that concrete could be mixed on location, difficult work on rocky terrain and a potential disruption of archaeological sites. Also, Bingel said, a solid wall could hamper border agents by blocking their view of the wall’s other side.

Setting aside the need for congressional approval and a likely fight with Mexico over financing, many who study borders doubt that a mass of concrete would accomplish its purpose. From the ancient Great Wall of China to Israel’s modern security barrier, walls rarely prove totally impervious to people set on traversing them.

Walls tend to be crude solutions to complex problems and are evidence of geopolitical failure, said Michael Dear, a professor at the University of California, Berkeley, who specializes in the border with Mexico.

“People always find a way to go above or below or through a wall,” said Dear, the author of “Why Walls Won’t Work.” “It’s just political window dressing and rabble-rousing of the worst order.”

Water supplies

What about the water?

Billions of gallons flow between the United States and Mexico, funneling lifeblood to farms and communities on both sides of the border. The Colorado River sends water south; the Rio Grande, a natural boundary for hundreds of miles, delivers precious water from Mexico, through dozens of canals, to much of South Texas.

Water experts in the Southwest question how Trump’s border wall could accommodate those crucial flows and still provide the barrier he wants.

Another complication is that a nearly 50-year-old treaty between Mexico and the United States prevents any construction that obstructs or diverts the flow of the waterways. The wall, in other words, could not interfere in any way with the flow of water in either direction.

“We’ve been joking: How big does the hole in the wall have to be to let the water run through it?” said Patricia Mulroy, a Brookings Institution senior fellow and former Nevada water official.

One option would be a wire fence, but that wouldcatch all sorts of trash.

“There are going to be consequences for any kind of fence you build,” said Gabriel Eckstein, a water law expert at Texas A&M University.

Asked whether the proposed wall might impede water flows, Hope Hicks, a Trump spokeswoman, did not directly respond. “The proposal speaks for itself,” she said, adding that details could be found on Trump’s website. But the site does not address water flows.

Complicated environmental science and a fraught diplomatic history await anyone seeking to determine how to build a wall that cuts off the flow of people without violating the nation’s water treaty obligations.

Experts on those subjects, in interviews, were skeptical that one would ever be built. They speculated that the financial and political challenges were too great, among other reasons.

With the will and the financial wherewithal, though, an engineering solution can be found, said Sally Spener, the U.S. secretary for the International Boundary and Water Commission, which manages the water relationship between Mexico and the United States.

But she acknowledged longstanding and delicate water-sharing issues between the United States and Mexico — a point emphasized by Michael Connor, deputy secretary of the Interior Department, who has worked closely with Mexico on those subjects.

A wall could be built that accommodates water flows, Connor said, but “the question becomes, when does it become economically significant to address those issues?”

(Officials from the Bureau of Reclamation, which reports to Connor, said they could not quantify the cost given that they had not envisioned or studied such a project.)

A bigger concern, he said, was the addition of a potential major wrinkle into the delicate bilateral diplomacy over water sharing. “Any proposal that would threaten that relationship or trust we’ve built would concern me,” Connor said.

Several treaties, particularly one from 1944, outline rules for sharing water from the Colorado River, the Tijuana River and the Rio Grande.

It is a complex system that involves water flowing down the Colorado River through Nevada and Arizona into Mexico, and down the Rio Grande from New Mexico into Mexico, where it is replenished by major tributaries before turning to Texas. (On the United States side, the Rio Grande watershed alone includes well over 4 million people.)

The relationship has been strained by drought: Tensions have flared over the last 15 years when Texas accused Mexico of delivering insufficient supplies of water.

The Texas Commission of Environmental Quality, overseen by appointees of the governor, cited a 2013 Texas A&M study saying that Mexico’s failure to deliver its annual water allotment cost the state economy nearly 5,000 jobs and $400 million each year.

Mexico countered that it did not have the water because of drought — a position that some American environmentalists said was valid.

The issue was resolved this year when the United States, jointly with Mexico, said the water debt had been paid off. Separately, the United States and Mexico took five years, from 2007 to 2012, to reach a landmark agreement to fix canals and water delivery systems to cope with historic drought.

As part of that agreement, Mexico agreed to share some of its water supply from the Colorado River so that the water could be retained in Lake Mead, near the Nevada-Arizona border. That situation worked out well, highlighting friendly but delicate relations.

“I was there, and I know the hurdles we had to overcome to assure them we weren’t manipulating the system,” said Mulroy, from Brookings.

“We are dependent on the Mexicans trusting us,” she said. “It’s an enormous diplomatic issue.”

With a wall, Mulroy said, “the chances of another cooperative agreement would be nil.”

© 2016 The New York Times Company