It’s hard to find anyone in Washington who knows border issues better than Alan Bersin. His unique perspective combines years of frontline law enforcement experience with academic knowledge and intellectual interest in the historical, economic and social forces that are at work at the borders of the United States, especially the U.S.-Mexico line.

Bersin became U.S. attorney in San Diego in 1993 and subsequently spent almost five years as President Clinton’s “border czar,” overseeing a border-wide crackdown on illegal immigration and drug smuggling.

During the Obama administration, he served in several key posts in the Department of Homeland Security, including as acting commissioner of Customs and Border Protection, the force of 58,000 employees that includes the U.S. Border Patrol as well as CBP officers guarding air, land and sea ports of entry. He later served as assistant secretary for international affairs and chief diplomatic officer at DHS, a job he left last month.

ProPublica sat down to talk with him about the history, politics, rhetoric and reality surrounding the border issues that are driving a fierce national debate during the first weeks of the Trump administration.

ProPublica: In the presidential campaign last year and in political discourse in general, the U.S.-Mexico border has consistently been depicted as out of control. How does that compare to the turf you have come to know during the past 25 years?

Bersin: When I began as U.S. attorney in San Diego during the Clinton administration in 1993, the border was in fact out of control. Illegal immigration was rampant. The federal government’s reaction, and the efforts of three administrations, gradually changed that. Over that period, the government was spending up to $18 billion a year geared to strengthening the border. We went from 3,000 Border Patrol agents to 22,000 agents today, more than 18,000 of them on the southwest border. There were massive investments in technology, air reconnaissance, sensors. This completely altered the border.

In 1993 and 1994, the Justice Department launched two operations: Hold the Line in El Paso and Gatekeeper in San Diego, the areas where almost all of the illegal crossing was concentrated because it was so easy to cross. The Border Patrol was able to get control of those flows. The strategy had two goals: putting more agents on the line to apprehend people and create a deterrent to crossing, and spreading the traffic out. A critical dimension was the construction of fences and barriers and walls along 700 miles of the 1,900 miles of the border. The type of barrier depended on the terrain. There is triple fencing in San Diego, and significant barriers in places like Nogales and Yuma, Arizona and El Paso and Brownsville, Texas. The idea was to restore the rule of law, to bring order to a chaotic situation. The results became more and more apparent. Crime rates went down in the border region. Today, the number of migrants crossing is at a 30-year low. That’s because of years of bipartisan work on this issue. Has it achieved a complete sealing of the border? No. But it has achieved equilibrium and more effective management. During the last 10 years we have also seen the beginning of joint border management with Mexico. In the course of 25 years, we have developed a constructive relationship with Mexico that was nonexistent before. During the last eight to 10 years there have been continued efforts which have resulted in a strategic alliance with the Mexicans and improved safety and security at the border.

A major contribution has come from the changing nature of migration. People should remember that Mexican migration is now at a net negative. More Mexicans are leaving through deportation, and voluntary return, than are entering the United States legally and illegally.

In part, that’s a result of our efforts on border enforcement. But it’s also because Mexico now has the 13th largest economy in the world. The Organization for Economic Cooperation and Development predicts it will have a larger economy than Germany by 2042.

The Mexican people are increasingly middle class, and Mexico has substantially become a middle-class society. This is true despite the significant poverty, and the class and geographic inequality that have deep historical roots. Part of this process of change, as was the case in our own country, involves a difficult battle against organized crime. Nonetheless, Mexico has become a robust democracy with a robust press and an active legislature. It has gone from being a sending country for migrants to a transit country, and increasingly a receiving country for migrants in its own right.

Not only are the numbers of migrants entering the U.S. at the lowest levels in a generation, but they are now largely Central American. Four out of five border-crossers detained in South Texas are Guatemalan, Honduran or Salvadoran. They are driven by violence and poverty in their home countries and the desire for family reunification.

Indeed, many of the illegal crossers who have entered the country in the last two years after being detained have actually been either unaccompanied minors or families who request political asylum. The ability of the smugglers to attract large numbers of families and unaccompanied minors is a function of the inability of our immigration court system to process asylum claims in a timely fashion.

So the key to responding to the increase in Central American minors and families is less at the border than in the immigration bureaucracy?

Yes. The difficulty is twofold. First, the law change during the Bush administration gave the Department of Health and Human Services a central role in relocating Central American minors in the United States. (HHS has an Office of Refugee Resettlement that is responsible for sheltering and processing migrant *****ren and *****agers.) HHS has implemented this law by reuniting *****ren with their families, many of whom had entered illegally. This has unintentionally made HHS the last link in the smuggling chain from Central America and created a legal incentive for the continued illegal migration of minors.

The second issue is the longtime lack of funding and resources for the immigration court system. Migrants come up and no longer seek to evade the Border Patrol, but are actually left at the border by their smugglers. And they seek out Border Patrol agents or Customs and Border Protection officials to surrender to them and request political asylum. That’s the way in which they get entry into a system that will eventually release them into the country.

Not because the system was designed that way, but because that is the practical result of an immigration court system that was never resourced by Congress. Notwithstanding the requests of the Obama administration, it was still not funded by Congress to be able to provide timely hearings and adjudication of immigration benefits.

If there were a rapid method of adjudicating claims, we wouldn’t see what has occurred on the scale on which it has — people paroled into the country with hearings set for two, three, four years in the future. Often, they don’t show up for their hearings. And often, during the time they are in the country waiting for the immigration hearing, they are having *****ren and developing community ties. They generate real reasons to claim a right to remain, even though they’ve never been given a legal status. The bipartisan failure to build an effective immigration court system capable continues. It’s part and parcel of the general observation that the immigration system is broken.

The opening salvo in President Trump’s campaign last year, one that came to define this presidency in many ways, was a promise to build a wall on the border and make Mexico pay for it. What do you think of that idea?

I think there’s no question that the barriers, the fences and in certain urban areas, the walls, have had an important effect in terms of increasing the manageability and the security of the border. But in fact as [Secretary of Homeland Security] General [John] Kelly acknowledged at his confirmation hearing, walls and barriers alone are insufficient to insure security.

As [former Homeland Security] Secretary [Janet] Napolitano pointed out, if you build a 50-foot wall, you’ll soon be confronted with a 51-foot ladder. You need a strategy that involves layered defense: deployed patrols, sophisticated sensor equipment, and surveillance from the air. That is what has had a positive impact over the last generation.

The judgment we have to make is whether a physical wall costing billions of dollars, or a further investment in Border Patrol agents costing hundreds of millions of dollars, will achieve the result we seek. And is the objective worth the costs?

Most people who live at the border or are familiar with the border know that a Berlin-like wall stretching from San Diego to Brownsville is not necessary. And the costs would be prohibitive. And there are places on the border, such as the Arizona desert or the open terrain around the Big Bend in South Texas, where Mother Nature has created her own barrier that is not easily passable. Or if you do pass through it, you are easily detected. All of this will be debated over the next few years, since I believe DHS has acknowledged that it will take at least two years to work through the details of any wall with Congress. During that period many more facts will be brought to bear on that decision.

This is not to say that there aren’t places where you could actually strengthen the barrier dimension of the layered defense. But the image and the costs of a Berlin-like wall or a Great Wall of China is something that the American people have not accepted to date.

To the extent that President Trump means strengthened border security, I am fully in favor of the idea that the rule of the law, secure borders and public safety should prevail. Drugs should not enter illegally. Migration should take place in accordance with lawful norms and secure and safe procedures. And in fact we should be working more with the Mexicans to prevent the flow of guns going south into Mexico that have fueled so much of the violence there, and the smuggling of cash and the money laundering that transnational criminal organizations have instituted in North America, including in the United States.

The symbolic issue of the wall cuts two ways. To the extent that it is interpreted as an insult to Mexico, especially the demand for reimbursement, it could do irrevocable harm to cooperation with Mexico that dates from the Merida accords during the Bush administration in 2006.

Over the years you’ve developed strong professional and personal relationships with Mexican security officials and diplomats. Are you concerned about the U.S.-Mexico relationship as the result of tensions with the Trump administration?

In the last generation we’ve moved past a U.S.-Mexico relationship that while friendly on the surface, and demilitarized for the most part, really was not a genuinely cooperative relationship. As a result of the U.S.-Mexico War in the 19th century, and the treaty of Guadalupe Hidalgo in 1848, half of what was Mexico was severed and became much of the western part of the United States. To add insult to injury, most Americans never knew that, and most Mexicans have never forgotten it.

The nationalism and the protectionism that was built into the Mexican Revolution in 1910 and that characterized the Mexican attitude to the United States for much of the 20th century were difficult to overcome. But that actually has occurred. And the cooperation and the trust and the confidence that have been built is not something that should be abandoned without great consideration for the potentially grave consequences to the United States.

The relationship is much stronger than people think. But it takes a great deal of care and cultivation. For example: The work the Mexicans are doing in terms of migration control on Mexico’s southern border is crucial to our own border security. Mexican enforcement efforts have become critical to moderating and mitigating the flow of Central American migrants at Mexico’s southern border with Guatemala. In the last two years, the Mexicans have detained nearly 400,000 migrants whose intent was to come to the United States. The Mexicans return the detained Central American migrants by bus or by air to the countries they come from.

A specific example: the notorious train known as La Bestia (The Beast). It was a great risk to many Central American migrants, but a primary migratory route. They rode on top of freight trains across Mexico, literally to the U.S.-Mexico border region, where they would get off and seek to be smuggled into the United States. Migration aboard those trains, which was a feature of the U.S.-Mexican smuggling landscape for many years, has been for all intents and purposes been stopped by Mexican authorities. They prevent migrants from getting on the trains and riding illegally. This kind of cooperation was unthinkable ten years ago and not even feasible five years ago.

A further example is the information sharing that takes place routinely between Mexico and the United States. Every air traveler entering Mexico is vetted against U.S. databases. The air passenger screening system Mexico has in place involves these checks against U.S. national security and criminal data bases. There are plainclothes U.S. officers stationed at airports in Mexico working with Mexican immigration officials to protect the United States. This joint security program has been in place for at least six years and is a huge asset.

As much as your focus has been border enforcement and fighting crime, you are enthusiastic about North American integration. Why?

Six million jobs in the U.S. depend on trade with Mexico. Ten border states — six in Mexico and four in the United States — combined have the third or fourth largest economy in the world. Twenty-nine U.S. states depend on Mexico as their primary export market. All of this is a function of the vibrant cross-border economic links that now exist between our two countries. We do nearly $700 billion a year in trade. Research by the University of California indicates that, absent this trading level between the two countries, the United States would have lost more jobs during the 2008 recession than it did.

We need to realize that the economic situation between Mexico and the United States is not just one in which we trade with one another. We make things together. We have shared production platforms. Cross-border trade is part of a single production process, and while apparently the Trump administration will seek to re-examine elements of that production platform, it is what it is and won’t be easily dismantled.

It’s not just a Mexican phenomenon, it’s also a Canadian phenomenon. You have auto parts being manufactured in Ottawa or in Detroit that are assembled in plants in Guanajuato or Queretaro. This is the way in which “Made in North America” operates today.

The cry of “Make America Great Again” reflects accurately that, after the fall of the Soviet Union, the sole superpower status of the United States is coming to an end. For the first time since the second World War, we are not the sole dominant economy in the world. In large part this is because of the success of policies followed by the U.S. to create an environment, a peaceful period in history in which economies could grow and countries could benefit.

While some in China and Europe and in this country think this embodies the decline of America, in fact that is not the case. The potential of Mexico, Canada and the United States is enormous. We have a combined population of half a billion people; peaceful trade-friendly borders that are the envy of the world; the prospect of energy independence is within reach and will change the geopolitical situation of United States; we do a trillion dollars in trade among the three countries; more than 18,000 American companies are involved in foreign direct investment in Mexico and Canada; an increasing number of Mexican companies are creating jobs in the United States.

If we continue in this direction, we would see a North America emerge that will be highly competitive worldwide both as an economic unit, but also in terms of security. Our security in a global world must be looked at on a continental basis.

That is not to say that President Trump hasn’t identified the losers in those propositions: People in our so-called Rust Belt have lost out and politics and society have not been responsive either in providing the kind of additional support they need or to retrain them for jobs that are being created in the new economy.

But we must recognize that this massive economic bloc that’s emerging in North America cannot be accomplished unilaterally. It must be accomplished in partnership with Mexico and Canada. And we have to work together to secure the continent in order to keep dangerous people and dangerous things out and strengthen perimeter security on a continental basis.

Let’s talk about other aspects of border enforcement. The big question: What do you think of the recent presidential executive order and travel ban on people from seven predominantly Muslim countries and the impact it could have?

The stated reason for the suspension, it’s not a ban, has been to provide an opportunity to do a review of the vetting and screening procedures to be sure that the full resources — intelligence-wise and data-wise — have been employed to prevent dangerous people from coming into the country.

I think this administration will come to appreciate that there have been enormous advances since 9/11 to build a very robust set of targeting procedures and watch lists to screen travelers coming to the United States. The National Targeting Center in Virginia run by Customs and Border Protection checks the background of every traveler who seeks to enter the country.

It works with airlines and foreign governments to stop high-risk persons from traveling to the United States. It uses data that has been gathered by our intelligence services and military all over the world, particularly in Afghanistan and Iraq, as well as from foreign governments. While no one can argue with strengthening these systems, this will always be a work in progress, they are already quite effective. Presumably our colleagues in the new Trump administration will find that to be the case.

With respect to the so-called travel ban, we have an order that appears to have been drafted at the White House without the weeks of advance review that would ordinarily exist in government agencies. We can already see the checks and balances asserting themselves: The first was Secretary Kelly reversing the directive that barred green-card holders from entering the United States. That was rolled back. But the prior review by career professionals, which is a basic check against both error and malice, did not occur here.

In fact, the speed with which the order was imposed suggested there was an imminent danger and an urgent need to do something drastic. Do you see a basis in reality for that?

I think the record is to the contrary. The comparison to what’s going on in Europe is instructive in terms of the ability of the Islamic State to put organized, trained and equipped terrorists to operate within the Schengen region of Europe. That contrasts to the situation here in the United States, where ISIS has had little success in doing the same. We have seen self-radicalized individuals doing terrible things to be sure, but that’s a different phenomenon.

As for refugees, the record is also clear in terms of the 18 to 24 months that are required to qualify a refugee for entry into the United States. There are a very stringent set of protocols and procedures administered by the United Nations High Commission on Refugees as well as the U.S. government. The notion that this was a wide-open gap in our security, and this executive order was necessary to close it, is just contrary to the facts.

Are you concerned about the repercussions of the images of babies and old women and interpreters who’ve worked for the U.S. military being turned away at our borders? What is the potential impact this debate could have at home and abroad?

There’s no question that there’s a risk. The efforts that are being made to counter violent extremism in our country require intensive work with Muslim communities in order to develop trust and confidence. I’m worried about the impact on these efforts of today’s political rhetoric, whether or not it gets translated into actual policy by the administration. These relationships depend on what happens long term, over months and months and the years to come.

And there’s a larger point here. Homeland security is inherently transnational today. There’s hardly anything adverse that happens in our homeland that doesn’t have a cause or effect that’s generated abroad. Increasingly, we must rely on our allies and foreign governments to share information and data to secure our country. The extent to which we cooperate with foreign governments is essential to the vetting that we’ve talked about. One of the dimensions the executive order requires is an assessment of that information sharing, and that is a positive development. I think the administration will find a considerable amount of information is already shared, although there is room for much improvement with many countries. We have to remember that information sharing is restricted by legal barriers and cultural barriers and by the notion that information is power and therefore should be hoarded so if you share information you can extract something in exchange. In today’s digital online world, those who don’t share information will be isolated and left behind. We need the data of other countries to connect the dots.

As a result of this reality, DHS has the third largest number of people stationed abroad among U.S. civilian agencies. We can’t defend the country by looking at the borderline as the first line of defense rather than as the last line of defense. We have to secure the flow of goods and people by engaging with foreign entities. We assure our security by securing the flows as early as we can before they arrive and as far away from our borders as we can. To do that, we have officials of CBP, the Secret Service, Citizenship and Immigration Services, Homeland Security Investigations, TSA and the U.S. Coast Guard stationed all over the world working with their foreign counterparts. These relationships are primarily matters of trust and confidence. Classically, what goes around comes around. We should be wary, particularly with our closest friends and allies, of breaking down the trust and confidence that lie at the foundation of these relationships.