READ ABOUT U.S. EFFORTS to seal the Mexican border, and you quickly encounter two words. The first is futile. Take this June 5 dispatch in U.S. News & World Report, which reports on the "deep sense of futility" about illegal immigration in the town of Nogales, on the Arizona-Mexico border. "The number of Border Patrol agents has increased more than 200 percent in less than 15 years.... Yet the number of people estimated to cross the border illegally each year has remained fairly constant, at about half a million. Says Pima County Sheriff Clarence Dupnik, `I don't think anybody, anywhere, knows what it would take to secure the border.'"

The second word is perverse. A May 7 article in the San Francisco Chronicle reports that "[m]any experts believe that the current pattern of illegal immigration from Mexico and Central America was a consequence of the 1986 law's border tightening--followed by a tougher crackdown in 1996 that built fences in San Diego and El Paso. `The perverse effect has been to dramatically lower return migration out of the country,' said Douglas S. Massey, a Princeton University sociologist ... by making border crossing `very risky and unpleasant and increasingly expensive ... you make it more likely that migrants ... just hunker down and stay.'"

These words have a rich pedigree. In 1991, the great economist Albert O. Hirschman published The Rhetoric of Reaction, a study of conservative arguments against progressive reform dating back to the French revolution. And he divided them into three types. First, conservatives argued that liberal reforms always came to naught. In the 1850s, Alexis de Tocqueville insisted that the French Revolution, for all its sound and fury, had actually made little difference. In the 1890s, Vilfredo Pareto insisted that universal suffrage doesn't change the basic distribution of political power. In the 1970s, Milton Friedman declared that anti-poverty spending had virtually no impact on the lives of the poor. Hirschman called this the "futility" thesis.

He also noted a second kind of conservative argument. In 1793, Friedrich Schiller argued that the French Revolution had indeed made a difference: By trying to "install the holy Rights of Man," it had done the exact opposite, plunging France "into barbarism and servitude." A century later, after Great Britain extended voting rights to the working class, conservatives predicted that this, too, would have unintended consequences: It would destroy good government. And, almost a century after that, conservatives made the same argument against anti-poverty spending: It wasn't simply useless; it was counterproductive. As Charles Murray put it in 1984, "We tried to provide more for the poor and produced more poor instead." Hirschman gave this argument a name as well. He called it the "perversity" thesis. Viewed through Hirschman's prism, today's immigration debate is downright bizarre. The United States doesn't allow nearly enough legal immigration to quench its massive thirst for cheap labor. Millions of Mexicans are willing-- indeed, desperate--to provide that labor. And the U.S.-Mexican border is over 2, 000 miles long. By the logic that conservatives have been using for hundreds of years, U.S. efforts to seal the border are virtually guaranteed to fail, backfire, or both. And yet it is today's "conservatives" who insist that it can- -and must--be done.

The hottest idea on the American right today is a wall, stretching from the Gulf of Mexico to the Pacific Ocean. As it happens, the United States has already experimented with border walls. In the 1990s, it built them in two of the places that were experiencing the most illegal immigration, San Diego and El Paso--and succeeded in diverting illegal immigrants into the countryside, particularly in Arizona, which is now seized by the same immigration panic that convulsed California a decade ago. The walls went up; the Immigration and Naturalization Service saw its budget triple between 1993 and 2002; the Border Patrol doubled in size. And, according to the best estimates, the number of people illegally crossing from Mexico into the United States remained roughly the same.

But the effort wasn't merely futile; it was also perverse. By sending illegal immigrants across deserts and over mountains, the United States made their journey much more brutal. Border-crossing deaths are up eightfold over the last decade; far more people now die trying to cross into the United States than died trying to traverse the Berlin Wall. To survive their arduous journey, illegal immigrants pay large sums to human smugglers--many of them connected to gangs that have used their newfound wealth to launch a crime wave, particularly around Phoenix.

And, if those unintended consequences are not bad enough, the border crackdown has also convinced many illegal immigrants to stay in the United States--for fear that, if they return to Mexico, they'll never make it back. Since the '90s, according to one study, the average illegal immigrant's stay in the United States has doubled. And, when illegal immigrants lay down roots, they place a greater burden on America's social service system. In Massey's words, that means "increased costs to society for schools, housing, and medical care. We've accomplished the very thing we set out to avoid."

But "conservatives" are undeterred. If the United States extends a wall along the entire border, they insist, we will finally solve the problem. Yeah, right. There's a wall near Douglas, Arizona. As the town's mayor recently noted, it is punctured routinely. Illegal immigrants "brought a portable metal grinder to cut through.... The fences may as well not exist to them." In recent years, American authorities have discovered dozens of tunnels under the border. One, running from Tijuana to San Diego, was a half-mile long, up to 80 feet deep, eight feet tall, had a concrete floor, and was wired for electricity. If Border Patrol blocks off all the tunnels, some experts predict illegal immigrants will start coming by boat. Or Mexican criminal gangs will turn their attention to far more sophisticated forging of immigration documents. When market forces push strongly in one direction, government efforts to intercede usually fail or backfire. For a long time, that idea has helped define what it means to be a conservative. Someone should tell today's Republican Party.

This article originally ran in the July 31, 2006, issue of the magazine.