Although some of this comically inadequate fencing is still in place, it has been replaced and augmented by modern barriers, first south of San Diego and then along much of the Arizona border and smaller sections in Texas. In compliance with the Secure Fence Act of 2006, there is close to seven hundred miles of fencing now along the two-thousand-mile border with Mexico, supported by technology such as ground sensors, pole-mounted remote cameras and even unmanned drones.

Combined with new tactics, the barrier in San Diego, consisting of two rows of fencing separated by a road for Border Patrol vehicles, had a dramatic impact on the flow. While San Diego accounted for more than 40 percent of all apprehensions in 1995, by 2011 it accounted for less than 13 percent of a much smaller total.

The Border Patrol’s new tactics were actually pioneered in the mid-1990s in El Paso, the other main crossing point for illegal immigrants at the time, and the decline there has been even more dramatic. In 1993, El Paso accounted for nearly 25 percent of all Border Patrol arrests. By 2011, it accounted for only about 3 percent (again, of a much smaller overall number).

Much of the traffic that originally went through these two locations shifted to Arizona, and the Clinton and Bush administrations were unprepared—or unwilling—to do what was necessary to respond. During much of the first decade of the twenty-first century, Arizona accounted for half of all Border Patrol apprehensions, helping explain the intensity of voter concern over immigration. Most of the Arizona border now has some form of fencing.

There have been significant improvements in enforcement away from the border as well. One of the most important elements of the expansive 1996 immigration law was the 287(g) program, which established a formal mechanism for integrating selected state and local police into federal immigration enforcement. An even broader program known as Secure Communities is on track to connect all police and sheriff’s departments in the nation to immigration databases so that every time a suspect is booked, his fingerprints will be checked against both the FBI’s records and those of the Department of Homeland Security. The target date for full national coverage is the end of 2013, but two-thirds of the nation’s local jurisdictions are already online, including all in the four border states and many others in the West, Midwest and South.

One result of these improvements has been an increase in the number of deportations—or “removals,” as they are now labeled. (Removals are distinct from “returns,” most of which involve Mexicans caught at the border and sent back across by the Border Patrol.) Although a comparison with past years isn’t exact because the Obama administration has added some categories of people to returns for public-relations purposes, the increase is still dramatic. In 1995, about fifty-one thousand people were deported (most of them illegal immigrants, but some legal immigrants who rendered themselves deportable by committing crimes). By 1998, the total had more than tripled to 175,000, and then it more than doubled again by 2008 to 360,000. It has remained close to four hundred thousand for the past several years.

Another vital area in which enforcement has improved is employment. Only in 1986 did it become unlawful for employers to hire illegal immigrants; before then, they were specifically exempted from the definition of “harboring” illegal immigrants by what came to be known as the Texas proviso. But even after 1986, the paper-based enforcement system was easily gamed and had little impact on illegal employment.

That began to change, very slowly, with the enactment of the 1996 law, which required the development of pilot programs to enable employers to electronically verify the information provided by new hires. The current iteration of these programs is called E-Verify, and employers use it to check the name, date of birth and Social Security number of new hires. While still voluntary, the system is having an increasingly large impact. More than three hundred thousand employers currently use it to screen new employees, and last year about a third of all hires nationwide were run through the system. Although the system is improving daily, it is still possible to fool it with a fully developed stolen identity. But most illegal immigrants don’t have the resources for that, and employers in industries with lots of illegal workers are finding that when they inform prospective employees they use E-Verify, many simply leave and look elsewhere.

Other enforcement improvements also merit attention. State driver’s license systems (including the nondriver identification cards issued by the state motor-vehicles agencies) are America’s national ID system—a decentralized one, to be sure, but a national system nonetheless. The weaknesses of that system became apparent a decade ago when the 9/11 Commission revealed that the nineteen hijackers had between them thirty state-issued pieces of identification, which they used to smoothly navigate American society. The result was the REAL ID Act of 2005, which established minimum standards for state IDs to be accepted for federal purposes, such as boarding airplanes.

Although the bill’s focus was preventing terrorist access to American society, any weakness can be exploited. Before 9/11, not even half the states checked a license applicant’s legal status. Today, only New Mexico and Washington State still issue regular driver’s licenses to illegal aliens, while Utah provides a special illegal-alien license. Although the REAL ID Act’s deadlines have been repeatedly extended to accommodate state objections, the genuine improvements in identification (including other innovations such as digitized birth records) have made it harder for illegal immigrants to live undetected in the United States.

Finally, states have passed a variety of measures that have contributed to the decline in illegal immigration. These include requirements that landlords verify the legal status of those applying for a lease and making the presence of illegal aliens a state offense. Even where these measures have been held up by the courts, the hyperbolic coverage of them in Spanish-language media outlets effectively persuaded some illegal immigrants to leave and deterred some prospective entrants from making the trip.

One state measure that survived court challenge has been shown to have made a notable difference. In 2007, Arizona passed a law requiring employers in the state to use the federal E-Verify system for hiring as a condition of retaining a business license. It was challenged by the U.S. Chamber of Commerce and various liberal and civil-rights groups, but it was finally upheld by the Supreme Court last year. Research by the Public Policy Institute of California found that in its first two years, the new law reduced Arizona’s illegal population by 17 percent—nearly one hundred thousand persons.

A weak U.S. economy and stronger enforcement are joined by a third possible reason for today’s relative lull in illegal immigration: changes within Mexico itself. The key development is the trend toward smaller families, thus slowing population growth and easing pressures for emigration.

The UN reports that Mexico’s total fertility rate (TFR), the number of children the average woman would have during her lifetime, was 6.7 in the early 1950s; in 1950, the population was about twenty-eight million. Over the next fifty years, that high fertility rate caused Mexico’s population to nearly quadruple to one hundred million at the turn of the century and to about 114 million now.

But as Mexico’s population exploded, the TFR began to decline. It is now estimated by the UN to be just over 2.2 (the CIA estimates 2.3). This is close to the U.S. rate of about 2.1, which is the “replacement rate” that a country needs for long-term population stability. What’s more, the UN projects Mexico’s fertility rate will fall to 1.7 by midcentury (about the current rate for Denmark and Finland), at which time Mexico’s population is expected to start declining.

OF THESE three factors—the U.S. economy, an enhanced enforcement regime and Mexico’s demographic shifts—the first is obviously the most changeable. The unemployment rate has already begun to decline, dropping from 9.1 percent in August 2011 to 8.2 percent in June of this year. GDP growth resumed in 2010 at an annual rate of 2.8 percent (though the 2011 rate was lower).

What’s more, there’s evidence that immigrants are capturing a disproportionate share of whatever new jobs are being created. A Center for Immigration Studies report looking specifically at Texas found that, from 2007 through the second quarter of 2011, 81 percent of job growth went to newly arrived immigrants, half of them illegal aliens.

The vicissitudes of the economy obviously can’t be relied on to limit illegal immigration. So to the degree that the economy is the cause for the current lull, it would seem to be temporary. That raises a question: Even with the return of strong job growth, would the new enforcement measures continue to blunt renewed pressure for illegal immigration?

President Obama’s answer would appear to be yes. Speaking in El Paso last year, he outlined various improvements in border enforcement, then mocked those still dissatisfied:

But even though we’ve answered these concerns, I’ve got to say I suspect there are still going to be some who are trying to move the goal posts on us one more time. . . . You know, they said we needed to triple the Border Patrol. Or now they’re going to say we need to quadruple the Border Patrol. Or they’ll want a higher fence. Maybe they’ll need a moat. Maybe they want alligators in the moat. They’ll never be satisfied. And I understand that. That’s politics.

Image: Pullquote: The Government Accountability Office reported last year that only 44 percent of the border is under “operational control.” It’s hard to say a task is complete when it’s not even half done.Essay Types: Essay