Roberto Suro is director of the Tomas Rivera Policy Institute and a professor of public policy at the University of Southern California. Jorge G. Castañeda, a former foreign minister of Mexico, is a professor of politics and Latin American and Caribbean studies at New York UniversityEveryone, it seems, is remaking the United States’ immigration system. The Senate and the House have their respective gangs of eight; labor and business groups have their talks; and the White House has its say, along with dozens of lobbyists and advocacy groups.But there is one critical player missing from the effort: Mexico. No reform can be successfully devised or implemented without the willing participation of the Mexican government and public, so why not get them involved from the start?That involvement needs to begin May 2, when President Obama visits Mexican President Enrique Peña Nieto. And it should start with Obama admitting the obvious: He needs help.Although many elements of an immigration bill remain unresolved, three objectives are essential: legalizing the current population of unauthorized migrants, creating an effective enforcement system that thwarts recurring illegal immigration and channeling future flows through temporary and permanent migration programs. None of these goals can be accomplished — let alone all three at once — without engaging Mexico as a full partner. About 12 million people born in Mexico live in the United States. They account for 30 percent of the foreign-born population. They are not going away. Rather, their numbers will grow.Despite a predictable downturn during the Great Recession, the U.S. labor market has not lost its appetite for Mexican workers. Even with a tepid economy, we can expect a net flow averaging 260,000 people, both legal and illegal, every year through 2017, according to a recent study by the Wilson Center and the Migration Policy Institute. That is almost back to the pre-recession level of 280,000 migrants a year. And the study concluded that if the U.S. economy lights up, particularly in the construction sector, the estimated net flow could reach 330,000 a year before the end of the decade. Moreover, the unauthorized population has proved remarkably resilient. Since Obama took office in 2009, more than 1.2 million people have been removed from the country, but new arrivals have taken their place. Despite the deportations and record numbers of Border Patrol agents, the Department of Homeland Security estimates that the number of illegal immigrants living in the United States has remained the same — about 11.5 million — for at least three years now. About 60 percent are from Mexico.The last time Washington tried a legalization program, in 1986, Congress limited eligibility to long-time residents and farmworkers, and the application process was an obstacle course. As a result, only about half of the unauthorized population received legal status. That left a big underground population, and all of the human networks and illicit businesses that facilitate unauthorized migration remained in place. The big lesson from 1986 is that partial amnesties don’t accomplish the long-term goal of eliminating illegal immigration. To succeed this time, the entire unauthorized population — minus dangerous criminals — needs to be eligible, and everyone must be brought under a legal umbrella as quickly as possible.Congress can require a long and hard road to eventual citizenship with language tests, background checks and employment requirements at various stages along the way. That is not a problem. The first stage, however, is critical. Ideally, all 11.5 million people would come out of the shadows and present themselves to government authorities so the process can begin expeditiously. Given the number of people involved, that would be no small undertaking. The federal government will need help from many quarters, including from Mexico.The illegal immigrants would be motivated to participate if they were protected from deportation. Obtaining basic civil rights, access to credit and the ability to look an employer in the eye are what they want most, and the process should give them those right away. Inevitably there would be hoops to jump through such as producing identification documents, proof of residency and employment, and money for fees. That is harder than it sounds if you think about coming to this country with only the clothes on your back and then living underground for years. The process must help immigrants jump through those hoops or else many potential applicants will be disqualified, discouraged or, worse yet, will resort to counterfeit documents. A successful legalization effort will require mobilizing organizations that immigrants trust, such as their community organizations and churches but also the network of 50 Mexican consulates around the country. Only a concerted effort by multiple Mexican entities can ensure that applicants have the necessary documents, information and legal advice. Mexico’s cooperation also will be essential to prevent a resurgence of illegal immigration across our shared border. The United States has more than doubled the size of the Border Patrol over the past decade and has built hundreds of miles of fences but still can’t declare that the border is secure. It is past time to engage Mexico more fully. Some successful efforts at cross-border law enforcement have developed on reducing the flow drugs and guns and could be extended to human trafficking. In the context of a broad reform that creates robust legal channels for Mexican migration, the Mexican government could also start enforcing its laws against traveling without the proper papers.Some of the biggest challenges for U.S. immigration control are even farther away, on Mexico’s southern border. More than 1.5 million illegal immigrants from El Salvador, Honduras and Guatemala live in the United States. By all indications, the flow through Mexico to the United States is increasing as those countries experience a wicked combination of criminal violence, economic stagnation and ineffective governance.Mexico is needed for the third element of immigration reform as well. The United States must undertake both legalization of the current population of illegal immigrants and new immigration enforcement measures on the border and in the workplace — but neither can be effective in the long run without adequate legal channels for handling future migration. The best way to end illegal immigration is to make it legal.Migration between Mexico and the United States is built on family networks and labor-market ties that are profound, enduring and efficient. The current era of Mexican migration developed in the 1970s, gained momentum in the 1980s, surged in the 1990s and remained closely intertwined with the U.S. economy in the 2000s. This migration now has an immoveable anchor in that about 3.8 million Mexican migrants, about a third of the total, are U.S. citizens, and 100,000 more take the oath every year — twice the number from any other country. They are fully American, but like many immigrants before them, they retain close ties to families and communities in their home country, traveling, sending remittances and doing business.Managing a well-established migration of this kind requires several types of admissions programs, some short term and some permanent, some centered on employment, others on family; the mix will need to evolve. Mexico’s cooperation will be essential for a temporary program of any significant size. Workers will need to be vetted and contracted in Mexico before departure, and Mexico will need to ensure that there are financial and employment incentives for them to return. Soon after taking office, the Obama administration proclaimed the doctrine of “shared responsibility” in fighting the drug trade; this notion has become the touchstone for cooperation on security policies between the United States and Mexico. Now it is time to see whether that sense of mutual interests and obligations can bring results on immigration. The last time the presidents of these two countries talked seriously about immigration was in 2001, when George W. Bush and Vicente Fox tried to negotiate a broad bilateral accord in which the governments would jointly administer immigration programs. The Sept. 11 attacks derailed talk of a special deal for Mexico, and there is no need to go back to the idea. Instead, Obama simply needs to acknowledge that when it comes to U.S. immigration policies, there is Mexico accounting for the lion’s share of the flow — and then there is every other country producing just fractions. He does not need to compromise any U.S. interests to accept the fact that Mexico is special. And when Obama meets Peña Nieto, he simply needs to recognize that the United States can’t fix its immigration problems without Mexico’s help.