The choice presented to undocumented immigrants must be clear-cut: If they've avoided serious criminality, worked steadily at jobs and mastered English, they should get the chance to pay stiff fines (at least several thousand dollars) for initial unauthorized entry and to pay all back taxes in order to qualify for legal residency and begin the path to citizenship. This approach would allow the immigrant to acknowledge his wrongdoing in violating the border, and to commit himself to making up for that wrongdoing by following rules that allow him to enter the mainstream.
For those who don't choose to pay the fines and obey the new regulations, or whose past criminality or failure to learn English makes them ineligible for "earned legalization," there's only one other choice: Go home.
The vigorous enforcement of workplace sanctions against employers —combined with a vastly improved, high-tech system of ID, and aggressive efforts to serve existing warrants and deport criminals —should greatly reduce the number of illegal immigrants who neither want nor deserve to remain in the USA.
Anti-immigration extremists will protest that no illegals deserve this choice: They entered the nation without permission, and so the only response is to demand their departure. Many activists in this camp (including Pat Buchanan) acknowledge the impracticality of mass deportations but favor a policy of "attrition": making it impossible for illegals to work by enforcing strictures against employers, thereby forcing them to leave the country. The problem with this scheme is that even its most enthusiastic advocates acknowledge it won't work for most of the undocumented.
In his book State of Emergency: The Third World Invasion and Conquest of America, Buchanan approvingly cites studies saying that with a policy of "attrition through enforcement," some "half the illegal aliens here could be persuaded to return home voluntarily within five years." But what of the other half —numbering more than 5 million? Pushed even further into the shadows, thrown out of work and continuing to defy all laws against them, they would become a far more dangerous, destructive population than they represent today.
In other words, even the most militant foes of illegal immigration recognize that they will manage to expel only half of the undocumented —and the ones they'd force out largely comprise the wrong half. People who want to work hard, get educations for their children and become Americans would, for the most part, be forced to depart. Meanwhile, those who stay would disproportionately represent lawbreakers and others who manage to operate "below the radar."
Provide motivation
Ultimately, even the most stringent effort to expel illegals will leave millions remaining in the USA, and the most important goal for this mass of humanity is assimilation —following the path of previous immigrants into the national mainstream.
At the moment, the biggest obstacle to such assimilation involves the impossibility of ever achieving legal residency. When the law makes it impossible for an immigrant to correct his illegal status, he faces greatly reduced motivation to embrace a new American identity.
That's why the only realistic policy comes down to divide and conquer: Divide the immigrants who ought to leave from those who deserve to stay, and then conquer the families who pursue legal status with the overwhelming power of the American dream that they've chosen to embrace.