Joseph Carens has advanced a strong moral argument in favor of amnesty for irregular migrants in the United States. I agree with the need for some kind of legalization program and share his ethical concerns. The current immigration crisis, however, stems from deeper U.S. policy failures that must be addressed, or the problem of undocumented migration will simply recreate itself.

The core of the U.S. immigration dilemma is Mexico. Of the roughly eleven million people in the United States with undocumented status, about 60 percent—some 6.5 million people—come from Mexico. The next closest case is El Salvador, with around 570,000 undocumented migrants, followed by Guatemala at 400,000; the numbers drop off rapidly from there. If we deal effectively with migration from Mexico, other immigration problems become small by comparison and much easier to resolve.

The roots of the Mexican problem go back to 1965, when the U.S. Congress ended a 22-year-old temporary worker agreement with Mexico and enacted a new cap on immigration from the Western Hemisphere. This measure was followed in 1976 by updated country-specific limits. In a few short years, Mexico went from enjoying access to 450,000 annual guest worker visas and an unlimited number of residence visas to having no guest worker visas at all and just 20,000 visas for permanent residence.

The number of migrants entering the United States from Mexico did not change very much after 1965. What changed was their legal status. Before that year there was no significant undocumented migration to the United States, but afterward the population grew steadily to reach an estimated five million in 1986.

The Immigration Reform and Control Act (IRCA) was enacted in 1986 to deal with the emerging immigration crisis in three ways: legalizing former undocumented migrants, tightening border enforcement, and criminalizing undocumented hiring. Despite the long history of Mexico-U.S. migration and the obvious demand for Mexican workers in the United States, the law made no provision for the legal entry of additional residents or workers.

The lack of provision for legal movement was especially counterproductive because Mexico and the United States were drawing together economically. By 1994 the two countries had signed a joint agreement to lower barriers to the cross-border movement of goods, capital, information, services, commodities, and certain classes of people. But within the newly integrated North American economy, the United States refused to recognize the movement of labor. Instead in 1993 and 1994 the Border Patrol launched a series of police actions to blockade the nation’s busiest border sectors.

The result was predictable. After falling to around two million in the wake of IRCA, the undocumented population quickly began to grow again thanks to the lack of legal avenues for entry. In response the United States further militarized its southern border, increasing the Border Patrol’s budget by a factor of ten between 1986 and 2002 and raising the number of agents fivefold by 2008.

In the context of ongoing economic integration within North America and continued labor demand from the United States, this militarization of the border did not reduce the number of undocumented entries from Mexico. What it did do was dramatically lower the number of undocumented exits.

Militarizing the border increased the costs and risks of undocumented border crossing, and migrants quite logically adapted to this new reality by minimizing border crossing. But not by deciding to remain in Mexico. Instead, they hunkered down in the United States once they had run the gauntlet at the border.

In response to tightened border enforcement, undocumented emigration from the United States was halved. By making no provision for the movement of workers within North America and by militarizing the border with our second-largest trading partner, U.S. policy did not merely fail—it backfired, actually doubling the net inflow of undocumented migrants to produce today’s population of eleven million.

Although legalizing undocumented migrants may be a moral imperative, an amnesty will not by itself solve the underlying problem of undocumented migration. Mexico is a trillion-dollar economy with 110 million people, and it is a friendly nation with which we are increasingly integrated socially and economically. Yet in terms of immigration policy, we treat Mexico like any other nation, allocating to it the same number of visas as to Botswana or Nepal. In the absence of legal means to accommodate the legitimate demands for work and residence visas, the flow has been diverted to unauthorized channels.

If undocumented migration is to be solved in the long term, we must address the realities of North American economic integration by providing for the legal movement of workers between Mexico and the United States. Increasing the number of permanent-residence visas and once again making temporary labor visas accessible to Mexican workers is the greater part of that effort.

This policy makes practical as well as moral sense, given that many jobs in the United States are seasonal in nature or do not provide earnings sufficient to support U.S.-based workers in a competitive global economy. Moreover, contrary to what most Americans think, the vast majority of Mexicans do not begin migrating with the intention of settling permanently in the United States. Instead they come to work temporarily in order to accumulate savings or generate remittances to solve an economic problem at home. If they had their druthers, most would return home after one or two periods of short-term U.S. labor. Militarizing the border with Mexico only frustrates the natural desire of migrants to circulate rather than settle, driving up the costs of immigration to the citizens of both countries.