When more than twenty million immigrants arrived in the United States between 1880 and 1920, the government attempted to classify them according to prevailing ideas about race and nationality. But this proved hard to do. Ideas about racial or national difference were slippery, contested, and yet consequential--were "Hebrews" a "race," a "religion," or a "people"? As Joel Perlmann shows, a self-appointed pair of officials created the government's...