An estimated 11 million people live in the United States illegally. Everyone agrees that this is intolerable, and -- deportation being impossible and possibly unfair -- Congress appears on the verge of granting them a path to citizenship. But legal reform is not going to solve the problem of illegal immigration. That's because illegal immigration is not really a problem, or if it is a problem, it is a problem that no one wants to solve.It is common to think that the huge pool of illegal immigrants reflects a failure of government. Congress has established rules that determine who gets in and who stays out but has failed to spend the money to enforce the law. The solution is more enforcement resources, symbolized by the huge wall being constructed among the Saguaros in the Sonoran desert.But the reality is that the United States has long been well served by a three-tiered system of immigration. The top tier consists of highly desired foreign workers, who are offered green cards, which typically lead to citizenship.The second tier consists of skilled and semi-skilled people who can obtain short-term visas, usually for three years. Some of them prove themselves while here and end up acquiring a green card as well.Then there is a third tier, typically unskilled people, who can be removed at any time and for any reason, yet are frequently permitted certain privileges, such as a driver's license. They are also permitted to work -- while in practice being denied the protection of employment and labor laws.We call these people "illegal immigrants" but that is a misnomer. Little effort is made to stop them from working or to expel them. And those who proved themselves by staying employed, learning English, and making enough money to afford a moderate fine, were given a path to citizenship in 1986, as may occur again if Congress passes immigration reform this year.Illegal immigrants do break the law, but they break the law in the sense that everyone breaks the law. Think of traffic laws, which everyone breaks but which are also only enforced selectively -- largely against people suspected of committing drug crimes or other misdeeds. The law against illegal entry is (sort of) enforced at the border, but hardly at all against people once they arrive, except if they commit serious crimes, in which case they are sent to jail and then deported.It is an open secret that illegal workers are, or have been, employed by some of the country's largest and most important companies, like Tyson Foods. Yet, the number of worksite enforcement actions -- where federal immigration authorities raid a worksite and drag away illegal workers -- has been minuscule.In 2011, worksite raids resulted in the arrest of 1,471 illegal workers out of an estimated 8 million. In the same year, only 385 employers out of 6 million were fined for hiring illegal workers. And this counted for an increase from 2006, when precisely zero employers were punished. In other words, the odds of being punished for participating in the illegal immigration economy are something like the odds of being given a ticket for driving 56 mph in a 55 mph zone.Despite the federal system E-Verify, efforts to force employers to check the status of job applicants have mostly foundered because of their cost and the risk that lawful residents will be mistakenly deemed illegal (though this is in fact rare). Which is just to say that we are unwilling to incur the enforcement costs because we don't actually want to enforce.What we have is a de facto quasi-guest-worker system, where foreign workers who overstay their visas or sneak across the border are permitted to stay and work as long as they do not commit a serious crime, look like terrorists or cause other trouble. In many places, authorities take pains to assure illegal immigrants that they will not be turned over to federal Immigration and Customs Enforcement so that they will cooperate with the police and social services.Anxious to attract new residents, Baltimore, for example, prohibits its employees, including police, from asking anyone about his or her immigration status. Of course, Arizona, which has suffered from violent crime associated with border crossings, has tried to crack down, but it is an exception.The system exists because it serves America's interests. Americans have a voracious appetite for unskilled labor -- in the form of nannies, gardeners, restaurant workers, agricultural laborers, construction workers and factory hands. And foreign countries contain huge pools of unskilled labor. Unskilled Mexican laborers would rather pick strawberries in the United States for a pittance than pick strawberries in Mexico that are exported to the United States, and for which they are paid even less than a pittance. U.S. businesses would rather pay illegal workers a pittance than Americans a pittance and a half.What is ingenious about our system is that it allows us to take advantage of unskilled labor at low cost; exile those people who cause trouble; and ultimately grant amnesty to those who prove their worth by working steadily, learning English, and obeying criminal law. They will leave on their own when unemployment rises and come back when labor is in demand. In this way, public policy recognizes a sliding scale of legal protections for aliens, offering the strongest protections to those we want the most, and the weakest protections to those we are less sure about.Why not recognize this guest-worker system in law? The bipartisan framework for immigration reform hints at such a change without explicitly endorsing it. But others have proposed this, and it was seriously considered in 2007 immigration reform negotiations.The idea of making our guest-worker system official is to move potential illegal workers into legal channels, where they can be tracked and also protected from exploitation. Liberals have long opposed any system that creates second-class citizens, but because liberals also oppose harsh immigration enforcement measures, they end up reaping the benefits of a pool of low-wage second-class citizens without calling them that. Unions oppose guest-worker programs because union organization requires a long-term commitment, which temporary workers cannot make, while they compete with union members for jobs.But these are not the real obstacles to a guest-worker program. Enshrined in law, such a system could solve the problem of illegal immigration only if it authorized the same low wages and bad working conditions that illegal workers currently accept. The demand for such workers is so high precisely because they lack legal protections and can be paid little and often treated poorly. The more generous the guest-worker program is, the more likely that it will be evaded.At the same time, however, neither Republicans nor Democrats will support a guest-worker program that permits foreign workers to be paid less than the minimum wage. And guest workers, like illegal immigrants, integrate themselves here and have children who become American citizens. It would be difficult to demand or force them to leave if they do not want to. In the end, they are not really guests.Here's a prediction. A path to citizenship will be offered to the current 11 million, and if it is not too onerous, most of them will take it. But others will not, planting the seeds of a new illegal population. Possibly a guest-worker program will be put into place, but even if so, it will be too small and too entangled with bureaucracy for employers and workers to want to use.Over the years, millions more people from Mexico and especially (as Mexico's economy continues to improve) Central and South America will illegally enter the United States. They will be partly drawn by jobs, and partly by waiting family and friends, and the law will not deter them because they expect that sooner or later another path to citizenship will open up.Ten or 20 years from now, everyone will recognize a new illegal immigration "problem," which we will again "solve" by removing the "illegal" label from the foreheads of the migrants and affixing the "legal" label in its place.Eric Posner, a professor at the University of Chicago Law School, is a co-author of The Executive Unbound: After the Madisonian Republic and Climate Change Justice.