The Black Hawk helicopter swooped down into the Arizona desert one day last week, and four agents jumped out to round up a dazed band of illegal immigrants, who hadn't expected arrest to come out of the sky. The raid was an early salvo in the Department of Homeland Security's new high-tech effort to seize control of the most porous stretch of the U.S.-Mexican border. When all the pieces are in place, probably by the end of the month, the $10 million Arizona Border Control, or ABC, initiative will deploy 260 new agents and a small arsenal of equipment -- aerial drones, airplanes, helicopters and ground sensors -- as well as additional prosecutors and tents to hold those apprehended trying to enter the country.

The department's immediate goal is to complete a crackdown begun more than a decade ago in California and Texas -- an effort that succeeded in closing off large segments of the frontier, only to funnel a vast flow of illegal migrants through Arizona. But ABC also points boldly to the future: Though not heralded as such, it is the administration's first serious step toward implementing the controversial immigration proposal President Bush outlined in January.

No, that's not a misprint or a misstatement. What the president proposed was a historic liberalization of U.S. policy: a loosely structured and uncapped guest-worker program that could radically increase the number of foreigners admitted each year. And yes, ABC is a tough-as-nails enforcement effort. But paradoxical as it sounds, the two go hand in hand. Indeed, we as a nation can't have one without the other -- can't have liberalization without enforcement, and can't have meaningful enforcement without liberalization.

Like its predecessors, the Arizona initiative will probably make a tangible difference in a limited area. The most successful of the earlier efforts, Operation Gatekeeper, implemented early in the Clinton years between San Diego and Tijuana, significantly reduced the numbers trying to cross in that region. Long stretches of double fencing, video cameras and ground sensors, a dramatic increase in border agents and a new strategy of higher visibility combined to put an end to nightly free-for-alls in which literally thousands of people would simply run from one country to the other.

Similar operations followed at other popular crossing points, and local Border Patrol units claimed victory, only to find illegal entries skyrocketing in more remote places such as Arizona. The difference is that the Arizona terrain is rougher and the distances longer, making the transit much riskier -- a migrant dies every day now in the Sonoran Desert -- and any effort to patrol the line much more difficult. ABC hopes to defeat this deadly geography with unmanned aerial drones to detect migrants in the desert and with more creative prosecutions of smugglers. But skeptics are already warning that the flow will simply be diverted elsewhere, while the new restrictive measures will fuel the growth of more sophisticated smuggling rings.

So, are our efforts to control our borders highly effective -- or utterly futile? The truth is, it depends -- because border enforcement is only part of the solution. Ultimately, we cannot hope to control the flow until we come to grips with the economic demand that lures foreign workers here in the first place. The last time we tried immigration reform, in the mid-'80s, we thought we could remove that magnet by imposing sanctions on employers who hired illegal workers, and we failed miserably. But that doesn't mean we can't succeed -- only that we must set a more realistic goal.

There can be no hope, in a global economy, of eliminating American employers' increasing reliance on imported labor. But we can and must do a better job of managing the demand: by meeting the bulk of it through legal means -- higher legal immigration quotas -- and then using our potentially powerful enforcement tools, both at the border and in the workplace, to keep the flow within those more realistic bounds. The best analogy is Prohibition. Imposing a ban on alcohol only fueled illicit activity and made a mockery of the law; recognizing reality and managing the demand allowed us to control drink very effectively with liquor licenses and import duties.

So, too, with immigration. It's currently all but impossible to impose any kind of sanction on the half-million or more people who come here illegally every year, even with stepped-up border enforcement. Aggressive prosecution of employers would require criminalizing whole sectors of the U.S. economy. And with the undocumented population now numbering 8 million to 10 million, even simple crimes such as document fraud can seem beyond the legal system's reach. The result is hugely demoralizing, for law enforcement and for the rest of us.

As during Prohibition, we invite a poisonous contempt for the law. Just ask employers with a foreign-born workforce about the I-9 employment eligibility form that they must fill out for every laborer -- knowing that most of the Social Security numbers they write in are fraudulent. Thanks to new technology and a better database, the Social Security Administration can now spot false numbers electronically, and last year it began alerting employers. But heavily immigrant-manned businesses and the local communities that depend on them rose up in protest, and the SSA backed off. In communities like that, even when we have the capability, we can't afford to enforce our implausibly restrictive immigration code.

Why do we have such an unrealistic, patently unenforceable policy? For political reasons, of course. For years now -- decades, in fact -- Congress has been passing stern-sounding immigration measures to reassure a public clamoring for control of our borders. The result: a militarized frontier and quotas far too low to support our vigorous economic growth. But at the same time, in instance after instance going back nearly a century, we have winked at those laws and routinely flouted them for the sake of businesses dependent on migrant labor.

Disingenuous as it was, the compromise worked reasonably well for a time, but the numbers have gotten too big now, and civil society is growing less and less tolerant of that kind of hypocrisy. The two conflicting demands aren't going to disappear any time soon. On the contrary: Demographic and economic trends make us increasingly reliant on immigrant workers, and yet the public is still insisting on intensified control of the frontiers, today more than ever. The challenge in this century will be to craft a more honest way to reconcile the two needs, combining liberalization with truly effective enforcement.

The means are available. Widening legal immigration channels should greatly reduce the illicit flow: All the evidence suggests that most migrants would prefer to come legally. But this will not eliminate the need for monitoring or sanctions.

The most promising monitoring tool is the electronic database. The Border Patrol's IDENT system, a computerized registry of arrestees and other suspects that was critical to the past decade's success on the frontier, is being linked to other digitized records from the State Department, the FBI and elsewhere to create the vast new U.S.-VISIT database against which federal agents will soon vet every foreigner who enters the country. Biometric identifiers and computerized lists are also likely to form the core of any new effort to check employer compliance, including participation in a temporary worker program. Random audits and occasional raids can be used to keep employers honest. And anti-racketeering measures developed to fight organized crime can help combat smuggling cartels.

As ever in such endeavors, the devil will be in the details. Privacy safeguards will be essential. Identification numbers -- such as Social Security or credit card numbers -- may turn out to be more effective than physical visas or worker ID cards. And arguably the most important breakthroughs will be psychological: for the public, accepting a dose of realism about immigration, and for business, understanding that the law is now going to be enforced.

Meanwhile, policymakers will have to look beyond the border to devise new alternatives, both in the workplace, where economic regulation may be as important as more conventional enforcement, and internationally, as we develop ways to cooperate with immigrant-sending countries. Along with the ABC initiative, for example, Homeland Security is expected to collaborate with Mexican authorities to fly apprehended migrants back to their home villages instead of simply returning them across the frontier -- a sensitive joint effort intended to pave the way for more critical kinds of cooperation. We mustn't kid ourselves: We will never have an airtight border. But the outlines of a new system are taking shape, and intriguing experiments abound.

The immediate question, predictably, is about politics -- how to get from here to there. We need to start, some reformers argue, by proving we can enhance enforcement, restoring public confidence in the immigration system before we even begin to think about liberalizing the criteria for admission. Perhaps, others counter, but we can't expect much more than marginal improvements unless enhanced enforcement is accompanied by a radical easing of quotas, whether with a guest-worker program such as the one the president is proposing or by some other means.

Which should come first -- the chicken or the egg? Can we make the leap of faith, or must we inch our way forward with efforts like the ABC initiative? The Bush administration, perhaps shrewdly, is trying both approaches. But one way or another, we need to cover the distance -- to replace our old unenforceable hypocrisy with an honest, modern answer that works. Tamar Jacoby is a senior fellow at the Manhattan Institute. Her most recent book is "Reinventing the Melting Pot: The New Immigrants and What It Means To Be American" (Basic Books).

Hang on there: Tougher enforcement has to be paired with realistic quotas.