IN THE SCORCHING Arizona desert near the Mexican border, small groups of men stand guard. They are armed with guns, dogs, binoculars, and high-tech surveillance equipment that allows them to transmit pictures directly to the Internet. They are self-proclaimed vigilantes, people who have taken the law into their own hands. They have vowed to stop illegal immigrants who try to make the dangerous crossing from Mexico each day to live and work in the United States. These vigilantes believe that the nation is under siege from immigrant groups and that the U.S. government is not doing enough to prevent it.

The existence of such people makes it clear that not everyone welcomes the changing face of the nation. Indeed, there are tensions and challenges that result when people of vastly different cultural, linguistic, religious, and racial backgrounds...