IN THE SMALL factory town of Morganton, North Carolina, the priest of the local Catholic church delivers his mass in English, Spanish, and Hmong, the language spoken by a growing community of immigrants from the Southeast Asian country of Laos. Just north of the towering skyscrapers of Dallas, Texas, the families of fifty thousand mostly African immigrants and refugees crowd into apartment buildings that were once home to white middle-income commuters. Visitors to New York City's heavily trafficked Canal Street find Vietnamese markets, Indian-owned electronics stores, and fast-food restaurants that serve everything from Chinese dim sum to Middle Eastern falafel.

From the great city of New York to small towns like Morganton, America is growing increasingly diverse. Americans today claim a wide variety of racial, religious, linguistic, and cultural backgrounds. In contrast to many nations of...