As the longest government shutdown in U.S. history drags on, one bone of contention is hogging all of the headlines: The U.S. border with Mexico. Discussions of the threat posed by an unsecured frontier and of the efficacy of border walls and other security measures have sparked fierce debate over how best to secure the boundary. Because the topic has spawned a great deal of interest – and perhaps just as much misinformation – a discussion of these issues is timely.

U.S. President Donald Trump’s administration created quite a stir when it attempted to frame the border issue as a crisis, citing the apprehension of many “terrorist suspects” at the border. But the numbers, quite simply, do not support the claim; indeed, no one who has illegally crossed the U.S.-Mexico border has ever gone on to perpetrate, much less even attempt, a terrorist attack in the United States. If anything, U.S. officials worried about a threat emanating from a land border should be looking north, rather than south. Over the past two decades, several high-profile plots have involved suspects entering the United States from Canada, including a plot to bomb the New York subway system in 1997, the “millennium bomb” plot around the New Year in 2000, the Toronto 17 case in 2006, and a thwarted plot to bomb Amtrak trains in upstate New York in 2013.

But beyond this, most of the people involved in terrorist attacks in the United States have been U.S. residents – either citizens or green-card holders – rather than operatives who have entered the country from overseas. And, like the 9/11 hijackers, those few foreign attackers traveling into the United States to conduct operations have generally held visas. There have been a few instances of terrorism suspects using fraudulent documents, like two of the 1993 World Trade Center bombing suspects, but a simple fact remains: Most involved in planning or executing terrorist attacks inside the United States have already been in the country.

Now, this is not to say that there are no compelling national security arguments for securing the U.S.-Mexico border — but a terrorist crisis is not one. The Trump administration appears to have grasped this reality, and it is noteworthy that the president made no mention of terrorism in discussing the border during his prime-time speech Jan. 8.

Rather than terrorists, far more criminal aliens cross the U.S.-Mexico border every year. Critics have vocally denounced Trump’s characterization of undocumented aliens as dangerous criminals, correctly countering that the vast majority of migrants are not coming into the United States with the intent to commit crimes, but merely to seek work. The problem is, however, that unauthorized migrants are not subject to a vetting process, making it impossible for authorities to distinguish those seeking better opportunities from those with nefarious intent. Indeed, because some malefactors (whether members of Mexican cartels or Salvadoran gangs like Mara Salvatrucha (MS-13) or just criminals who operate alone) view the United States as a land of criminal opportunity, it is not uncommon for authorities to arrest and deport criminal aliens on multiple occasions, only for them to return.

And although U.S. officials are detaining far fewer undocumented migrants at the border now than in recent years, it nevertheless takes a great deal of time and effort for personnel from Customs and Border Protection and Immigration and Customs Enforcement agencies to process the undocumented aliens they do apprehend and sort out the criminals. Ultimately, an overall reduction in the flow of undocumented migrants would allow them to direct more effort toward apprehending the criminal aliens.

Aside from a few open-borders activists – who are, incidentally, instigating and facilitating migrant caravans to the U.S. border from Central America in an effort to foment a crisis on the U.S.-Mexico border – most reasonable people would agree that there is a legitimate need for both Mexico City and Washington to control their frontier. This, however, raises the question at the heart of the matter: What is the best way to secure the border?

Building a Wall

“Build that wall” was an oft-repeated mantra at Trump’s campaign rallies ahead of the 2016 election, but the slogan has since persisted at Trump events. As I discussed shortly after the 2016 election, walls do serve a purpose, but it will take far more than such a barrier to halt border crime.

As in any good security plan, there is a place for physical security barriers like fences, walls and doors. But in the absence of personnel who can effectively monitor these barriers and respond to any breaches, a determined intruder can always overcome a wall given time and opportunity. As history shows, people have succeeded in scaling, cutting, smashing, bending and tunneling under existing border walls. Fences and walls have been in place on the U.S.-Mexico border for decades now, restricting or, in many cases, redirecting the flow of contraband and people. In many cases, effective walls have shifted flows of people to areas without them; since the areas without fences tend to be more remote and in hostile environments, this has resulted in the deaths of more migrants, thereby discouraging many from attempting the trip.

But while these periodic barriers help stanch the flow of lower-value drugs like marijuana, as well as undocumented aliens, it does little to halt the influx of high-value drugs, such as cocaine, heroin, fentanyl, and methamphetamine, which kill tens of thousands of Americans every year. That’s because these illicit goods enter the United States through normal points of entry as part of the massive stream of legitimate products that pass across the border every day.

Subterfuge facilitates some of this smuggling, but it is also connected deeply to corruption. Smugglers can easily afford to pay a border official a few thousand dollars to wave through a half-million-dollar shipment of drugs. Likewise, wealthy migrants can afford high-quality counterfeit documents or the money to pay a bribe in exchange for genuine, but fraudulent, immigration documents such as visas, passports or border-crossing cards. As we’ve long noted, the better border security becomes in terms of effectiveness, the more that people become the weak link in the chain. There is a long history of corruption among local, state and federal authorities on both sides of the U.S.-Mexico border that testifies to this fact.

So, yes, walls can be effective if properly placed and monitored by security personnel. But it would be prohibitively expensive to patrol a wall that spans the length of the entire U.S.-Mexico border, especially in the most remote and environmentally hostile areas.

Installing Sensors

Americans seem predisposed to attribute magical qualities to technological solutions to problems – something that has been seen time and again in the border security discussion. However, the basic limitations that pertain to walls also apply to sensors designed to detect activity at the border. The U.S. government has been using cameras, radars and seismic sensor systems along the border for decades. Perhaps the most famous of these attempts was the so-called “Smart Border Initiative,” under which the government installed numerous sensors in the wake of the 2006 Secure Fence Act. But despite the millions of dollars of investments, the sensor systems have not offered a silver bullet on border security.

Like walls and fences, sensors are only as effective as the people who respond to any trigger. If agents can’t deploy rapidly to a location to apprehend a group of aliens or drug smugglers, all the sensors are doing is documenting the threat. While infrared and thermal camera technology is far superior to what it has been in years past, heat, fog, sandstorms, heavy rain, snow, and just plain old component failure can still play havoc with them. Additionally, animals are also responsible for many false alarms.

There really is no single, simple solution to border security. Walls and sensors certainly have their uses, but they also possess limits. Any real meaningful solution to border security must involve a holistic approach that involves not only security measures on the frontier but also attempts to decrease the demand for cheap labor and deadly drugs. Until the demand side of the equation is addressed, border security measures will reduce and redirect the flows, but they won’t stop clever and innovative people from finding the inevitable chinks in the armor.