Not for the first time, the world finds itself in an age of great movements of peoples. And once again, the United States is confronted with the challenge of absorbing large numbers of newcomers. There are approximately 200 million migrants and refugees worldwide, triple the number estimated by the U.N. only 17 years ago. In the United States alone, about a million new immigrants have entered every year since 1990, bringing the total immigrant population to more than 35 million, the largest number in the nation's history. Though Americans take justifiable pride in our history as a "nation of immigrants,"...