The United States was built on the shoulders of immigrants - millions from all over the world, including English Pilgrims, Spanish adventurers, African slaves, and Chinese laborers. The manpower these immigrants provided fueled the industrial expansion of the XIX Century and even at that early stage made the United States into a leading global economic power.

Throughout America's history, new immigrants have provided an economic underclass, performing menial and unskilled jobs. As they worked themselves up into the middle class and a better life for themselves and their families, they were replaced by still newer immigrants continuing the cycle. Even today many menial and unskilled jobs are filled by immigrants.

This immigration had two main drivers. One, of course, was the pull of America, the attraction of the Land of Opportunity. The other was the push of dysfunctional societies at home, leading millions to abandon families and familiar surroundings for the frightening and exhilarating trip to a strange and distant land. The industrialized world in general and the United States in particular served as safety valves for countries unable to care for their own populations, and benefited greatly from this flow of energetic, hard-working, ambitious youngsters.

The benefits to the United States were of course at the expense of the countries of origin, losing the hard working bodies that should have been building their own nations. Mass immigration is a symptom of dysfunctional societies. And the countries lose not only bodies, but brains. For years, many of the best and brightest from around the world came to the United States, earned their college degrees here, and stayed here to give an additional boost to the US economy. In the aftermath of the 9/11 attacks when visa regulations were abruptly tightened, there was widespread dismay at the loss of such talent. The net result of such Brain Drain remains controversial. On the one hand, US reliance on international medical school graduates has reduced the supply of physicians in many lower-income countries. On the other hand, the international movement of educated people changes the incentives to acquire education, sends enormous quantities of money across borders, leads to movements back and forth, and can contribute to the spread of trade, investment, technology, and ideas. And in recent years, there is an opposite concern of Brain Rain - foreigners getting advanced education in the United States and these educated brains then raining back on their home countries, increasing their competitiveness with the United States.

Globalization has changed all this with two powerful trends.

• US economic expansion has slowed dramatically, and with it the demand for unskilled labor. The ladder of upward mobility has shrunk; the Land of Opportunity has become a Land of Stagnation. Now there is a permanent underclass of workers who have little expectation of working their way up in society. This is complicating relations with Mexico and other Latin American nations which can no longer depend on the US economy as their safety valve.
• Faraway places are not so far away any more. In the networked world, every one is connected to every one else. The United States can no longer be an oasis of prosperity in a world of poverty. We are all in this together.

The core challenge of the XXI Century is to fashion a world of prosperity and avoid a world of turmoil. The United States is the only nation positioned to lead such an effort to global good governance. The US economy itself is transitioning to a steady state condition, prosperity without growth. It must avoid internal unrest by returning to a more equalitarian past. It has an opportunity to build satisfying and rewarding lives for all its citizens, but it cannot be divided into an entrenched upper class and a growing underclass.

US prosperity is coming to depend on the promise of prosperity everywhere. Under these conditions, mass immigration indicates a problem, not an opportunity to be taken advantage of. There will of course always be people attracted to the unique qualities of some other society and seek to move there temporarily or permanently. Such cultural or intellectual migration enriches all nations involved. But if it does not balance out to something close to zero, then there are challenges somewhere that must be addressed. Globally, the United States cannot work to build up nations at the same time it is draining their best human assets and it can no longer serve as a universal safety valve. The solution to widespread illegal immigration lies in working with the countries of origin to develop their own economies, while re-fashioning our own economy so that all jobs provide a living wage and there is no dependence on illegal workers. There can be no permanent underclass.